Good question. Long post but here are my thoughts.
There are some democrats voter that just believe whatever the Democrat politicians say (though there are Republican voters that do the same). I think a lot of people on the left are skeptical at whatever Trump says because he does have a reputation for stretching the truth. The mainstream media has a bias, hence I think it is good to get a variety of sources. See this link that explains why the 2020 elections are not fraudulent
https://www.vox.com/2020/11/13/21563825/2020-elections-most-secure-dhs-cisa-krebs. If you want a Conservative news site see National Review
https://www.nationalreview.com/. It's very conservative but more factually correct.
I'll address some concerns HKers may have about the election integrity.
1. Turnout was way higher: Trump is a polarizing figure you either love him or hate him. This drives turnout, democrat voters want to get rid of him really badly, Republican voters want to keep him in office. Generally, Democrats do better in high turnout elections, Republicans do better with low turnout elections (Republican voters are just more reliable). Mail-in ballots lead to increase turnout and due to Covid, there was an increase in the accessibility of mail-in ballots.
2. The integrity of mail in ballots: Mail-in ballots are safe and require signature verification and to register to vote one needs to present ID. Democrats vote by mail, Republicans vote in person, this is a trend that goes way beyond Trump. Day of voting ballots were counted first which is why Trump was up in WI, MI, PA, GA and later Biden passed him. I will admit on election night I thought Trump would probably win as I did not think Biden would not catch up in any state besides PA. Let's also remember in a state like AZ, Trump narrowed the gap a lot on Biden as counting continued.
3. Rural vs Urban vs Suburban: Democrats do well in Urban cities, Republicans do well in rural areas. Suburbs are shifting to Democrat but were very republican just 10 years ago. The Urban and suburban population in the US is growing, the rural ones are not. Compared to 2016, there are more voters in Democrat areas in 2020 while the Republican one has not seen much change at all.
4. Election officials are anti-Trump: GA has a Republican secretary of state (who is in charge of overseeing elections) so he wants Trump to win and Republicans to do well in GA. AZ governor is a Republican as well.
5. Republicans nationwide did well: Republicans gained seats in the House of Reps and the Democrats will have a very small majority. Republicans had to defend multiple senate seats and they had a net loss of 2 so far. The runoffs in GA should favor Republicans because runoffs have lower turnout (GA is still a Republican state so Republicans have the advantage; their candidates are also the incumbents). Trump doing worse than some Republican candidates down ticket shows that there are voters who don't like Trump but lean towards Republicans.
6. In 2016 Trump won WI, MI, PA by narrow margins: Biden did not need to do too much to flip those seats back as the margins Trump won them in 2016 were very small.
Finally, Trump can ask for recounts and go to court over allegations of voter fraud. If his campaign provides evidence I will be the first to admit I was wrong as I want a fair election.