Here's my recollection from civics class:

1. Each state makes their own laws about whether and when a dead candidate can be replaced on the ballot. It's entirely possible that citizens in some states would vote for dead candidates, while citizens in other states are voting for different candidates.

2. Remember you're actually voting for the electors that are listed underneath each (now presumed dead) presidential candidate. States make their own laws on how strictly the electors actually have to vote for the guy they pledged to. In many states, there's no penalty for being a "faithless" elector, and so in this scenario I would imagine that many electors in this situation would exercise their prerogative to vote for a living person who is eligible to hold the office. Perhaps even a re-nominee from their party.

3. Electors meet in their state capitols to sign and transmit the real ballots to Congress. However they decide to vote based on the above, I think it's important to the constitution that this happens by a deadline (December 18?), and if that is missed, we may be in real constitutional crisis territory.

4. After meeting, swearing in new members, and electing a house speaker (I really hope they can elect a speaker), Congress meets in a joint session to count the real ballots from step 3. This process is in the constitution. This is the process that was interrupted by a mob on Jan. 6, 2021. Under the constitution, Congress can only go against what the ballots say if there's a tie, in which case the house votes for president on a one state, one vote basis.

4a. As seen in 2021, Congress could conceivably engage in a deliberative process to determine whether ballots that are presented are real, legitimate ballots or not. The constitution doesn't actually allow them to do this. This is a real weakness in the system. If Congress isn't allowed to adjudicate ballot legitimacy, then who is? Apparently no one.

4b. There's also no alternate process if the majority of elector ballots have elected a dead person. Presumably Congress is required to declare that person President.

5. Constitution also says the next president's term starts on Jan. 20, 2025, at noon eastern, period. No ifs, ands, or buts. If Biden dies in office, his VP Kamala Harris will assume the duties. But that term ends on this date, and the fact that she ascended would have no direct bearing on whether she appears on any presidential ballot.

5a. This turnover deadline literally has no exceptions. If it is missed for any reason, we're probably in real serious constitutional crisis territory.

5b. If, per 4b, Congress declared that a dead person was elected president, I imagine that person would have a hard time showing up to inauguration on Jan 20. In this case I think that once the turnover has occurred, the normal line succession, which is 17 people deep*, can be used to swear in the next president.

5b1. In this scenario, next up would be the vice president-elect from the elector ballots, probably sworn into the vice presidency first. Third in line is the speaker of the house (I really, really hope they can elect a speaker). Then President Pro Tempore of the senate (the longest serving senator), then cabinet secretaries.

*Most of the 17 are cabinet secretaries. These do not expire automatically when the presidential term expires on Jan. 20. This means that all of the old president's secretaries are still in the line of succession (unless they foolishly submitted resignation letters) until a president is sworn in and either fires them or makes them resign.

**The presidential term starts at a precise date and time, but a president must swear the oath of office. Both things are in the constitution, so what happens when the time arrives, but the new president isn't sworn in?