The Electoral College meets in the various states to cast their ballots that will officially declare President-elect Biden the winner of the 2020 Presidential election, but that isn't the final word. That lies with Congress, who will come together for a joint session on January 6, 2021 to officially certify the Electoral College vote. The occasion is typically a quiet formality, but if these past four years have taught us anything it's that quiet formalities aren't in President Trump's DNA.
Here's what happens when the House and Senate meet to certify the election: The two houses meet in the House chamber on January 6, 2021, three days after the 117th Congress is sworn in. If one Congressperson and one Senator join forces to dispute a single (or dozens) of electors then the two houses must retreat to their chambers to debate for no more than two hours where the challenged electors will be voted on separately in each chamber.
If the two chambers disagree then that's when things could get tricky. It's never happened since the passage of the 1887 law called the Electoral Count Act which established our current system for certifying Presidential elections, but the vast majority of Constitutional scholars are confident the matter then goes back to governors in the disputed states the power to certify the electors, which would almost certainly benefit the Democrats.
Ok. All the "worst case scenario" stuff is now out in the open.
Here's why it's extremely unlikely that Congress will overrule the Electoral College: In short - the math simply isn't on Republican's side. Sure, one Senator can join forces with a House Republican to force each chamber to go into debate, but that is almost certainly going to be more about theatrics than anything else.
Republicans will only be swearing in 51 Senators to Democrats 48 due to a runoff election in Georgia (Senator Kelly Loeffler, who is also facing a runoff, will still be sworn in because she is currently filling in for former Senator Johnny Isaacson, whose term ends in January 2023).
Add in the fact that Republican Senators Lisa Murkowski, Susan Collins, Mitt Romney, Ben Sasse, and Bill Cassidy have all publicly called Joe Biden the President-elect and you can see that any effort to swing the Electoral College vote Trump's way is far more about theatrics than anything else.