The DNC's plan is becoming clearer:
1. Mr. Biden needs to leave office before the 2022 mid-term elections. Either by death or medical incapacitation.
2. Under the 25th amendment, Ms. Harris would become POTUS.
3. Also under the 25th amendment, Ms. Harris will name Hillary Clinton as her successor for VP. This is where "before the mid-term election" becomes important, because such a nomination needs to be confirmed by a majority vote of both houses of Congress.
4. Ms. Clinton is confirmed and becomes VP before the mid-term elections.
5. Ms. Harris declines to run for POTUS in 2024.
6. Ms. Clinton, now VP, announces her candidacy for POTUS in 2024.