There are a number of theories about the election, not all of them equally good. Some should be discarded at once; others merit investigation.
Here is a quick sketch of a view. Feel free to opine in the comments about others, or about why you do or don't agree with my list.
Dismiss:
"Scorecard"/"Hammer" theories. These are nonsense.
Auditing Georgia's absentee signatures. The President keeps talking about this. It's a great idea in principle, but in practice it's impossible because they set up the system to separate the envelope signatures from the ballots and discard the signed envelopes. There's no way to audit this aspect of the election, just as they intended, the scoundrels.
Take Seriously:
Dominion. This one has robust bipartisan support, at least if you abstract from the current moment and look at recent years. It drew a great deal of hostile coverage from the media, including from PBS; Elizabeth Warren was hotly opposed to it. Also, Wretchard thinks the system is fundamentally insecure, and he's a tech-centric guy and one of the smartest writers out there.
Georgia uses Dominion for the non-absentee votes, by the way, which means it ought to be a bipartisan issue even in the current moment. The Democrats would like to win the January 5 Senate runoffs, and how can they have confidence that the vote will be fair in a Republican-held state with a Republican governor and Secretary of State, and no less a governor than Kemp, whom they already regard as a chief voter fraud agent?
3 AM Election Drops: There were apparently several of these, all massively to 100% in favor of Biden, often in numbers sufficient to overcome Trump election-night margins. So too similar stories of corruption attested-to in sworn affidavits by people who were there to know what they saw.
Wild Turnout: Turnout was high this election, but some places it was far higher than in others. That is worth a look.
Joe-Only Ballots: These look a lot like fakery. Who stands in line, or goes to the trouble to apply for a ballot, and then votes in only one race? One goofball, ok, but 95,000 of them in a single state?
Statistical Evidence: Trump won all but one bellweather county; he won Florida and Ohio and North Carolina; the Republicans won all 27 'toss-up' Congressional races. He increased his vote count by 10 million, doubled his percentage of the black vote, and increased among Latinos strongly enough to win Florida and the border of Texas on the strength of their votes. Democrats almost lost the House of Representatives and are skin-of-their-teeth in having a Republican Senate too.
That kind of thing suggests Republicans had a really good year. Trump is personally hated by many, but not generally by Republicans. Supposedly Biden won on the strength of turnout -- but Trump had people standing in freezing weather, in a pandemic, for massive rallies every city he went to. If the story is to be believed, Biden had no coat-tails for a guy who drove the biggest turnout ever; Trump had coat-tails, but no coat.
Just a running list of thoughts for now. Feel free to add, or try to subtract.