It's very likely that if Covid-19 hadn't happened, he would've won in a landslide.

Technically, Trump did win. He was ahead of Biden on election night. Trumps lead was upended the next day by massive fraud, however. Biden owes his victory to Dominion, which stole the election.

It's hard to tell what lesson the GOP have learned from Trump, because he did do well in the 2020 election and there are clearly tens of millions of Americans who like his rhetoric at least.

The Republicans have learned that they don't have what it takes to be in power. They had the Presidency and the Senate for 4 years, and did nothing with it except cower under incessant media attacks. They caved under Democrat pressure time and time again, and abandoned their own principles just to get along. The Republicans had no loyalty whatsoever to Trump or his populist movement. They shied away from controversy and spent 4 years with their thumbs up their asses.

Gradually, they began to resent Trump for all the bad publicity he gave them, and they decided to turn on him when the time was right. The moment came on January 6th, with the protest at the Capitol. The Republicans lent their voices to the Democrat cause by labelling the protest as an insurrection, and by accepting the fraudulent electoral votes for Biden. In doing so, they betrayed their constituents and made themselves immensely unpopular.

He gained a bit among non-white voters (especially Hispanics in Florida and Texas), but lost white male voters (as a %) in the crucial swing states that he won in 2016.

Thats right. Trump alienated some of his white supporters by refusing to put down the BLM and Antifa riots, and by endorsing the Platinum plan for blacks. They were rightly offended by this blatant racial pandering, which rewarded blacks for their support, but did not reward whites for their support.