Hopefully he has many years left

Yes. Harris would be sworn in and be president.

If I understand the 20th Amendment, the job goes to the Vice President elect, however the Electoral College is supposed to cast its votes on December 14th, so it would be an intriguing hypothetical. If the President elect died before the 14th of December, who would the Electoral College vote for? Electors have been known to jump ranks, and in a close election, only a few electoral votes would be needed to change everything.

Of course, the Electoral College votes must also be ratified in January by a Joint Session of Congress, presided over by the current Vice President--more opportunity for mischief! If the President elect can hang on until after the January 6 ratification of the electoral votes, then it would seem clear that, in the case of the death of the President elect, the Vice President elect is to be inaugurated on January 20th. But I'm just a poli-sci guy with no constitutional law training, so what do I know?

That's a good question.

Don't think it's ever happened.

That would be a mess, I don't know what would happen.

He won't die. We all may not live to see January 20th 2021. Wear a mask. Good question tho. If the president-elect dies after Congress counts the votes and before the inauguration, the 20th Amendment would take care of things. It states that the vice president-elect, would step into the president-elect's shoes and assume the role of president-elect.

Harris becomes pres. No brainer , sheesh.

Good ?. It's possible, but highly unlikely. I won't be surprised if a few months after the inauguration that he steps down for a "personal health emergency", namely his dementia. I really doubt he will serve a full term.