How did allowing mail-in voting favour Biden over Trump?

Do you mean because Trump demonised mail-in voting for months before the election, and many of his followers believe COVID is a hoax?

Trump didn't "demonise" mail-in voting. He stated that it enabled voter fraud by eliminating chain of custody. I have yet to hear a single advocate of mail-in voting explain how chain of custody can be preserved and ballot harvesting prevented. There is clear evidence of harvesting, e.g. 100K ballots 99% for Biden, dropped off at a polling station in Philadelphia with no chain of custody, no Republican poll watchers allowed in the room, nothing downballot i.e. only Biden was selected on the ballots - this is ballot harvesting, at a very minimum.