Then we have POTUS Biden with Pelosi waiting in the wings.
Then POTUS Biden will receive instructions to nominate a certain Hillary Rodham Clinton for VPOTUS and does so. She will sail through the confirmation process. Three months later, POTUS Biden will succumb to a mysterious, virulent, and fatal follicle infection, elevating the devastated Clinton to POTUS.
If it's after the inaguration. Different rules apply before then. Consitutional rules.
If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.
Amendment XX, Section 3, Constitution for the United States.
Who knows how it would go if The One, were disqualified between the election, Nov. 4, and the casting of votes by the electors (Dec. 15th). They'd be free to vote for whomever they chose, which would likely mean whomever the DNC "annointed". Probably would not be Biden. I hope.