Question

First of all, thank you very much for the enlightening content on the site and in the books.
If I understood correctly, this is the essence of the cosmological argument:
There is some being/existence in the world.
Everything familiar to us has a cause.
The existence of being/existence also has a cause.
So that we do not get stuck in an infinite loop, we are forced to assume that at the head of the chain stands a cause that is not one of the things familiar to us.
That cause is called God.
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I did not sufficiently understand the meaning of this proof. After all, even at its conclusion we are forced to admit that we do not know, since we have no clue what that unfamiliar thing is that is not subject to the a priori logical rules that require a cause for everything. What compels us to say that we do know what the cause of the universe is, except that it is among the things that are not familiar and not intelligible to the minds of human beings, rather than simply saying that we do not know how to answer the question of what the cause of the universe is?
Seemingly, the answer the Rabbi would give me is that in truth we do not know how to answer the question of what the cause is; we only define that cause. But if so, I do not really understand where the argument with the atheists is here. I am quite sure there are many atheists who do not claim that there is an infinite regress here, but simply claim that they do not know. They only object to attributing the cause to God in His familiar meaning. But if God is only the name of the cause, it seems to me that everyone should agree to that. No?
(In the physico-theological argument I can understand that its purpose is to prove that there is some intelligent design here, because the other possibility held by atheists is that there is not. But regarding the cosmological argument, everyone agrees that there is a cause, and everyone agrees that we have no idea what it is, so where exactly is the argument here?)