Mail-in ballots?

[Biden his time]

Why do mail-in ballots typically lean more Republican in the Western United States and more Democratic in the Eastern United States?

This notably could be seen as when the mail-in ballots arrived in the 2020 U.S. Presidential Election (If I recall correctly), they shifted the results of Pennsylvania, Georgia, Michigan, and Wisconsin far enough to the left to flip what had been early leads for the Republican Party. New York state and Virginia, two blue states initially began the first few days with very low margins for Biden which heightened over the coming weeks. Meanwhile mail-in ballots made the state of North Carolina much closer than would be initially anticipated based off of early returns.

On the other hand, in the Western U.S., Arizona started out with a massive margin for the Democratic Party which narrowed significantly as the night went on. California likewise went through the same (though nowhere near enough to make the state competitive) pattern through the night (and currently is going through it right now once again as the recall election progresses).

Is it because mail-in voting was already widely used before the pandemic throughout the Western U.S.?

Will this pattern continue into the future? What can we expect?

[Stuart98]

Mail ins skew older usually. In the east this was offset by Republicans scaring all their voters from using them, but this effect was much less pronounced in states that had been using mail-ins for a while.

[TML]

In states with substantial Native Americans living on reservations, mail ballots tend not to benefit such Natives as much as rural whites, since such Natives often don’t have deliverable mailing addresses (their residences on reservations are often not registered with the postal service as deliverable). This was reflected in Montana in 2020 when the state facilitated mail-in voting statewide: although there was an increase in turnout across the board, the counties which had Native American majority populations had the lowest turnout rates while rural white-majority (and mostly non-college educated) counties had the highest turnout rates, greatly contributing to Republicans winning every single statewide race that year by decisive margins.

[Skill and Chance]

In states with universal or near universal mail in voting, the Republican in-person election day preference is often expressed by dropping it off or mailing it in day-of.  The early mail in votes were still incredibly Dem in the West, and you saw the same pattern of Republican improvement in the very last batches of absentee votes from the Eastern states with the most relaxed mail-in rules, like NC and IL. 

[NotSoLucky]

Mail ins skew older usually. In the east this was offset by Republicans scaring all their voters from using them, but this effect was much less pronounced in states that had been using mail-ins for a while.

I hate how Trump turned Mail in voting into a partisan talking point. Washington and Oregon have used mail voting for years without any issues, and yet Trump and his cronies keeps spreading a bunch of lies about how "fraudulent" it is