Online account succession is the practical question of what happens to each individual account when the person who opened it dies. There is no single answer, because every provider sets its own policy and those policies range from a built-in legacy contact feature to a support process to no mechanism at all.
Several major platforms now offer inactive account or legacy contact settings, letting you nominate someone in advance and specify whether data should be shared or the account deleted after a period of inactivity. Where these exist they are by far the most reliable route, because the permission is granted by you rather than argued for later by a grieving relative.
Where no such feature exists, the fallback is a support process requiring a death certificate and proof of authority, handled by staff following a script. Outcomes vary and timelines are unpredictable. Many providers will close an account but decline to hand over its contents, which is a meaningful distinction if the contents are what your family actually wanted.
The complication underneath all of this is that account terms often characterize the relationship as a non-transferable license. That framing means the account may simply end rather than pass to anyone. Where the value is in the data rather than the account itself, exporting an archive periodically is more dependable than relying on any succession process.