“Can anyone tell me how people know how many ballots were submitted by dems or repubs?”
How you vote is a secret. How you are registered, and if you voted is public information.
They are looking at public records.
To add to that, assumptions can be made based on “enthusiasm” measures. That means about 90% of Rs stay in line. The Biden enthusiasm was not nearly that high. But the “Trump haters” are enthusiastic.
What we should not dismiss are the independent vote. Those People can be scary.
So it's useless as the exit polling... for media results. I'd lie to their faces if they asked me.