My take is that Democrats lucked out with the timing of this election. Their strategy of continuously insulting and making strawmen out of Trump and - crucially - his VOTERS is unbelievably self-destructive. This seems to be a problem especially with media personalities and such.
If the election had been held in January this year Trump's landslide victory would have made it into history books.
Thankfully the pandemic and Trump's general idiocy made him unelectable for many educated voters who nonetheless hate what many democrats stand for at the moment.
Biden himself did a fine job, I think. Ultimately Bernie and Joe collectively will most likely have done just about enough to edge this.
Unlike Hillary, Joe had the common sense to not speak condescendingly to potential Trump voters.
P.S. If mainstream american media had adequately covered the Hunter Biden laptop thing Trump would also have won, probably. In that sense Democrats made their own luck by exerting semi-authoritarian control over media. Well done, but dangerous as hell moving forward.