The monotheism is stuff like this:
Advaita: Rig Veda says God is ‘ONE’ and God is Atman, then why believe and worship any other God in place of real God.+*****
The Vedas are not a theologically descriptive text that tries to purposefully outline the nature of God purposefully, so much as it is a record of ritual prayers and recitations which contain many epithets and names and qualities described by these praises and associations. Verses throughout the Vedas seem to indicate that these various names are referring to One, or aspects and attributes of One, but its focus isn't seemingly on proving that but just collecting the ritual recitations.
It is not like the Qur'an, its not the same sort of thing really, the Qur'an seems to be focused on actually teaching people something and making them conform to it, while these people already believed whatever they believed and these were some of the rituals they wanted to put down so that it remains recorded.
"i. 1. 1. a For food thee, for strength thee! b Ye are winds, ye are approachers. c Let the god Savitr impel you to the most excellent offering. d O invincible ones, swell with the share for the gods, Full of strength, of milk, rich in offspring, free from sickness, from disease. e Let no thief, no evil worker, have control over you. f Let Rudra's dart avoid you. g Abide ye, numerous, with this lord of cattle. h Do thou protect the cattle of the sacrificer."
That is how the Yajurved starts in the document I linked to, and you can see its the beginning of some sort of prayer that provides people with two names Savitr, and Rudra, which also can through reading the rest of the Vedas and Upanishads and commentaries, collect enough quotes and evidence which indicate that these various names were calling upon One or different attributes of One Power, just like how the Qur'an uses different names all in one sentence or paragraph even to describe Allah or call upon Allah's attributes.
Nowhere, as far as I recall, do the Vedas spend a great deal of time focused on making sure its entirely clear and fully understood that Monotheism is the right way or anything, so one has to go through and find little quotes here and there among the prayers that are being recited.
"b Since Viçvakarman is mighty in mind, Disposer, ordainer, and highest seer, Their offerings rejoice in food, Where say they is one beyond the seven Rsis. c He who is our father, our begetter, the ordainer, Who begot us from being unto being [1],"
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"The one god producing sky and earth Welds them together with arms, with wings. l What was the basis? Which and what his support? When producing earth [4] Viçvakarman, all−seeing, Disclosed the sky with his might. m What was the wood, and what the tree, Whence they formed sky and earth? O ye wise ones, inquire with your minds On what he stood as he supported the worlds. n Thy highest, lowest, Midmost abodes here, O Viçvakarman, In the offering do thou teach thy comrades, O faithful one; Do thou thyself sacrifice to thyself, rejoicing. o The lord of speech, Viçvakarman, Let us invoke this day to aid us [5], thought yoked for strength, May he delight in our nearest offerings, He with all healing, to aid (us), the doer of good deeds. p O Viçvakarman, waxing great with the oblation, Do thou thyself sacrifice to thyself rejoicing; May the others around, our foes, be confused; May our patrons here be rich. q O Viçvakarman, with the oblation as strengthening,"
Right before that, and right after that, are statements that "faces and arms and hands and feet on every side" and "you made Indra", while Indra is the same One God, and the stuff about body parts everywhere should only be understood as non-literal, referring to omni-presence, animation in all directions, animating all things, and being all places and behind every sort of creature and motion, yet, its very much misleading and confusing, which is why the language of the Qur'an is better.
So if your question was "what does the Yajurved say about monotheism", it contains quotes like all the Vedas which can be used to make it appear monotheistic and refer to One God even by a variety of names or names placed as symbolic figures next to each other, but its focus does not seem to really be on educating people in this regard as much as it is a series of ritual recitations, incantations, things that people recited for their community or their patrons. The Upanishads seem to be more of an educating text, one that tries to elaborate upon and explain things, and often does so by indicating Monotheism, possibly Monism, that there is only One God, and really One Thing also maybe.
Yet, people use the same verses or other verses to try to make there be numerous other Gods or divide all these into various Gods, some even say then that the "gods" are angels actually, but none of that seems to be the case. Terminology is used that seems to refer to natural things and aspects of nature or society and life, but these can also be taken more extensively as symbolic things in reference to qualities of God.
"c You Indra chose for the contest with Vrtra, ye chose Indra for the contest with Vrtra. d Ye are sprinkled. e I sprinkle you agreeable to Agni, to Agni and Soma. f Be ye pure for the divine rite, for the sacrifice to the gods. g The Raksas is shaken off, the evil spirits are shaken off."
The reason I keep showing these quotes is that I'm trying to explain that its not a text about demonstrating monotheism to students or learners or anything mainly.
"i So great art thou, thou art life, bestow life upon me; thou art strength, bestow strength upon me; thou art the yoker; thou art radiance, bestow radiance upon me. k To Agni, lord of the house, hail! To Soma, lord of the forest, hail! To Indra's strength hail! To the Maruts' force hail! I The gander seated in purity, the bright one seated in the atmosphere, The Hotr seated at the altar, the guest seated in the house, Seated among men, seated in the highest, seated in holy order, seated in the firmament, Born of the waters, born of the cows, born of holy order, born of the mountain, the great holy order. i. 8. 16. a Thou art Mitra, thou art Varuna. b May I be united with the All−gods. c Thou art the navel of kingly power, thou are the womb of kingly power. d Sit thou on the smooth, sit thou on the pleasant seat. e May she hurt thee not; may she hurt me not. f Varuna, of sure vows, hath set him down In the waters, with keen insight, for lordship. g O Brahman! Thou, O king, art the Brahman priest, thou art Savitr of true instigation. O Brahman! Thou, O king, art the Brahman priest, thou art Indra of true force [1]. O Brahman! Thou, O king, art the Brahman priest; thou art Indra, the kindly. O Brahman! Thou, O king, art the Brahman priest; thou art Varuna, of true rule. h Thou art the bolt of Indra, slaying foes; with this subject to me. i This king hath surmounted the quarters. k O thou of good fame! O thou of prosperity! O thou of true rule!"
You see? "You are" and then listing all these names and everything, saying you are Agni, Varuna, Mitra, All, Brahman, and a bunch of other stuff.
This explanatory commentary of the sacrifices is included as well:
"He offers to Agni on eight potsherds; the sacrificial fee is gold. (He offers) to Sarasvati an oblation; the sacrificial fee is a calf. To Savitr (he offers) on twelve potsherds; the sacrificial fee is a speckled (ox). To Pusan (he offers) an oblation; the sacrificial fee is a dark (ox), To Brhaspati (he offers) an oblation; the sacrificial fee is a white−backed (ox). To Indra (he offers) on eleven potsherds; the sacrificial fee is a bull. To Varuna (he offers) on ten potsherds; the sacrificial fee is a great castrated (ox). To Soma (he offers) an oblation; the sacrificial fee is a brown (ox). To Tvastr (he offers) on eight potsherds; the sacrificial fee is a white (ox) To Vishnu (he offers) on three potsherds; the sacrificial fee is a dwarf (ox)."
"He who practises witchcraft should offer to Agni and Visnu on eleven potsherds; Sarasvati should have a portion of the butter, and to Brhaspati an oblation (be offered); in that there is (an offering) on eleven potsherds to Agni and Visnu, and all the gods are Agni and the sacrifice is Visnu, with all the gods and the sacrifice be practises witchcraft against him; Sarasvati has a portion of the butter; Sarasvati is speech; verily with speech he practises against him; the oblation is Brhaspati's, Brhaspati is the holy power (Brahman) of the gods; verily with the holy power (Brahman) he practises against him [1]. Him who practises witchcraft they practise then against; he should double each of the Puronuvakyas, for special employment."
Now its possible that these developments were (most likely) included as notes later on, but regardless, it seems clear to most that the purpose of these texts is not for theological explanations overall, though they are used for that by some via extraction from things that are said during the suggested recitations and rituals.
So when I read through these, what I'm looking for are names, epithets, descriptions, praises, which might be useful. I consider all these One (even as it says, even as the article tries to show in some quotes, and there are more that I've seen which may be less overt so not selected as the main ones Monotheists use), so that Vayu refers to God, Indra, etc, and they refer to the All-God but also each may be used to refer to certain attribute or aspect, so just like we call Allah Mighty and Wise, and if we are seeking wisdom we might use the Wise epithet, and if we are seeking strength or protection, the Mighty epithet, invoking and calling upon that area of power of Allah, and it can be interpreted or used now in the same way that if someone wants something that they mentally associate with strength and consider the term Indra having to do with that, they can call upon Allah as Indra, Brahman, Varuna, Mitra, whichever seems appropriate or apt, and if it is a word that means something natural, one should modify their understanding to not mean the literal physical thing or version of that, so that when we say Noor, we hopefully don't mean literal physical light.
So, reading through the Vedas, and the Yajurved, its just ongoing praises and invocations and prayers and ritual recitations, with little clues and things that can be picked out, but not an ongoing discourse on the Oneness of Allah or anything like that as its focus.
