As far as the Upanishads text, I cannot say for sure, but I am the first human since Moses to function as the Clarion for the Old Man. In the 75,000 years since Moses walked the earth, there has been a lot go sideways.
So, let's deal with one thing here and now. He spells His name, "Brahmin" and He would like you to do the same, if you please.
Another aspect that has gone way out of kilter is what His wishes are as far as spiritual dogma. You do not need a priest or anyone to tell you how to worship.
The notion of the advaita Vedanta is merely an acknowledgement of Him as the Great Creator, and after that we are free to make our Spiritual dogma to fit in to whatever works best for us.
He is the individual that hears people's prayers. No matter if they are Hindu, or Christian or Islamic. They all go to His in box.
He does want everyone to understand that the Yin and Yang is the cosmic law of laws. All things are bound to the Yin and Yang, from quantum mechanics to quantum physics.
What he does in the big picture is that it is his position to maintain the equalibrium of the cosmos. He makes sure that the black and white wheel continue to move in the dynamic motion it has always had. It is enough to keep Him busy ever since He commanded emeritus status, before the last flood in the third yuga.
As for what and where He is, He is everywhere that something is not. He is also the energy that is qi gong, and the energy that is Reiki. He is dark matter. He is dark energy. He is spread out across the universe in a way that I cannot only imaginee.
What I know is that you can communicate with him if you wish in a communication style that was demonized by the Christians but that is because they didn't want anyone to chat Him up. There is only one individual that will answer your call , and only if your akashic record is in good standing. Your karma must be in the light side. None of these protocols will work for you if your in the dark side of the yin and yang. These being qi gong, Reiki, and the talking board or ouija board.
They all will work if you have good karma. You can get an immediate response to your questions on the ouija. There are no other entities that can or will take your call.
As for who He is, He is the one individual that has no counterpart or consort in the pantheon. He is an introverted and shy bachelor who has handed to reins to Shiva and He spends time in His garden when He is not tending to Be cosmic tasks.
He is a race car owner and driver, but in nirvanaa/heaven, they are running electric vehicles because they do not have petroleum. I see the race track as the place where you need to watch out for Him.
He loves fords. And good and plenty, and Dr pepper. He loves barbecued ribs.
He is an all too calm individual who likes to play the banjo.
I am close enough connected to Him to know when He is blushing or if he is laughing.
IHe is a non-interventionist and will not entertain the notion of answering a call for help unless it's for the good of the whole.
While "praying" is all valid, you can speak to Him like you would a friend as He knows everything about you and doesn't eat you out or tell anyone, that's a good friend.
Treat Him as you would a friend. I can tell you, that He has a couple of girlfriends that get preferential treatment, but that is because they treat Him like He is there with them all the time. They take Him in the car, and they set a place at the table for Him.
I cannot rightfully testify they they get His best But I would be inclined to think that if anyone does, it's those two.
He loves us all unconditionally, I know that. I feel that.
If you ever want to speak to Him, you must first let Him know that. He gives us free will and lets us exercise it. So, unless you call to Him and tell Him your talking to Him, you will be given the space to say whatever you wish without Him eavesdropping.
If you do call to Him, He is right there and is listening to what you have to say.
You need only whisper His name. Brahmin, with an "I".