You'll see a lot of early voting statistics out there that cite the partisan makeup of early voters, like, say, a D+15 among absentees in Florida, based on registration.
But remember this: the presidential preference of these voters is even more skewed than the party reg.

This works both ways, to be clear: if a hypothetical early vote is D+15 by reg and the election day vote is R+15, then the early vote might be Biden+25 and the election day vote might be Trump+25 (just as an ex.)

Basically, the reg Dems/GOP/Ind voting early are all better for Biden than those who intend to vote on Election Day, and vice versa. If you prefer: the force drawing Dems than Reps to mail is also luring a relatively Dem set of Reps/INDs, and the most Dem set of Dems, and so on.

So on Election Night, if you see D+15 in a state's early vote, you should absolutely not be surprised if it comes in at Biden>30 or whatever. And you shouldn't be at all surprised when the GOP comes roaring back and closes it right back up

That's exactly the right feeling lol https://twitter.com/big_chupa_cabra/status/1317468288369176576?s=20