In very short and simple term Trump undid himself when he and his close aids launched a fierce campaign against 'mail-in ballots'. They repeatedly denounced mail-in voting system and poisoned the minds of thier supporters against it even though it was clear to all that Americans were going to embrace it due to the advent of covid-19.
Trump supporters who couldn't come out to vote due to one reason or the other practically stayed away entirely from voting because thier hero (Trump) told them that the mail-in ballot system wasn't reliable.
While Trump was holding series of interview and press conferences denouncing mail-in ballots, Joe Biden invested so much time and energy praising the system and convinced his supporters to embrace it and take advantage of it.

On election day after the count of physical votes Trump was clearly leading. However, things turned sharply around when the mail-in votes started coming in. So far, Biden has worn over 80% of all mail-in votes in states that are traditional Republican states. So far as it stands Joe Biden has won already, they are just waiting for the maths to prove it.

Trump let himself down, he fights all even when he is supposed to be turning enemies into friend.

Biden has a lot of job to do to sanitize the once-admired and impeccable American political system that has become largely polarized under uncle Trump