Do exit pollsters start collecting data during early voting?
In mail in ballot states, are exit polls even taken?
To find out how turnout was among demographics, are exit polls the only source, or is that data compiled officially?
On your question 3, if by “officially” you mean by a city/county/state, I don’t recall ever seeing that kind of data, although I’ve never looked specifically for it either.
State and local officials can report turnout by precinct, but they don’t have access to demographic data other than what can be inferred from location. That all comes from exit polls, which are commissioned privately by news organizations and parties.
Except according to adaher’s link
In Georgia, at least, they do measure early voting by race. And possibly North Carolina, unless CNN is making an inference based on location.