Do you think the mailed in votes favored democrats because of some property of mail in voting, or because republicans had been discouraging their voters from using it for much of the preceding year?

I can’t tell if this is satire — can you clarify your point?

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.