NoM, the difference is that Biden is not physically capable of being president. He’s got serious dementia, which is why he’s being kept as much out of public as possible.
GHT, the DNC bylaws say the vacancy is filled by a special meeting of the full DNC, voting under its usual rules.
The RNC bylaws say the RNC can fill the vacancy or it can call a new convention; if it does not call a new convention, then at the meeting to fill the vacancy each state delegation gets the same number of votes as that state would have at a convention.