The whole question can be answered with the following fact.
Muhammad thought that the Torah was a revelation, or scripture, that God sent from Heaven to Moses.
He did not know that the only writing YHWH commited and delivered to us humans was the 10 Commandments.
This somehow eventually created a thought that God sent 3 books to the earth.
The Torah, The Zabur (Psalms of David), and the Injiil (one book called the Gospel).
If you read the Quran and Hadith, you will find that Muhammad claimed that there was 4 books sent to mankind.
This was the mentioned Tora, Psalms of David, and the Gospel, and he added the Quran.
Well, he did not know anything about the Torrah, except for distorted versions he obviously heard from Jewish folklore.
He also said that David could recite the Psalm of David in the time it takes to saddle his horse. Totally unaware of the other 149 Psalms.
The Injill to him was another book that was sent by God to Jesus.
As time passed, the educated Muslimss tried to reconcile the differences, and contradictions between the Quran, and the other 3 books.
The only argument they could come up with was to say that the Christians corrupted the pure revelations God sent to humans.
This is why Islamic scholars today call Textual Critisizm of the Bible "evidence" that the original books sent by Allah, dont exist anymore, and the Christians wholesale corrupted what we have today.
Unfortunately, the evidence that the Bible we have today, is virtually exact to 99.98% to what Muhammad had, and the evidence in support is all the manuscripts discovered over the past 250 Years proving that the Bible was already canonised 300 years before Muhammad's birth.
Now, the Quran NEVER CLAIM THAT THE BIBLE WAS CORRUPTED, BUT EVEN TELLS MUHAMMAD TO GO TO THE CHRISTIANS IF HE DOES NOT UNDERSTAND SCRIPTURE, AND ALLAH REVEALED THE TORA, PSALMS AND GOSPEL, AND NO ONE..
PLEASE NOTE...
NO ONE CAN CHANGE THE WORDS OF ALLAH.
The above is evidence that Muslims accuses their god that he was not allmighty in protecting his revelations, and they make Allah out as a liar.
I call this the nail in the Islamic coffin.
Then we had all these new religions shaping out of Islam, with little or no knowledge of the Bible and Jesus, and they again continued with the idea that there was more books sent down from heaven to mankind.
Not only in Islam, such as Drew Ali and his 7 circle quran, but the exact same as with Joseph Smith in 1825 with the Book of mormon.
The secret to unmask these people, is in Deuteronomy 18:18 to 22.
As them to give a prophecy from God, and they will give you stuff made up long after this so called prophet died.
And as for Jesus and for instance Moses, they were doing miracles daily, giving prophecies, and everyone knew what it was God speaking, and not them.
Greetings