Politically, Biden's dropping out creates all sorts of uncertainty.

Procedurally, however, what happens how is extremely clear: The DNC will use its convention to officially designate its nominees for president and vice president, and those people will appear on the ballot in all 50 states, DC, etc.

A summary of state ballot access laws, from the officials who run the elections, is at https://www.nass.org/sites/default/files/reports/summary-ballot-access-laws-president-june-2024.pdf

It would have been much more complicated if this had occurred after the convention.