Biden will win when all the votes are counted, but Trump is going to declare himself the winner tonight. The mail-in ballots don't even start getting counted in most battleground states until tomorrow morning and the overwhelming majority of Democratic voters have already voted. The ones showing up in person today are mostly Republicans, which means you're going to see all those tossup and "leaning blue" states as red for the next few days. During that time, Rs will claim they won, and demand that all ballot counting be stopped. This exact same thing happened in 2000 in Florida, and now 3 of the judges involved in that decisions are on the Supreme Court.
The only thing I don't know is what Dems will do in response. Could be they push back and get all the circuit courts involved. Could be what they did in 2000 with Florida, which is fukking nothing. But we're not gonna hear a Biden victory speech for a while, and Rs are going to be going apeshyt all week.