Selfhosted

Don't think it has WebDAV support but you can use rsync to back up to pcloud. That's how I handle it.

Also, by default when you upload files to Immich it creates it's own directory structure but they have Storage Templates you can enable/customize to make it more human readable so the backups are more useful if looking at them without Immich.

Another option of you want more control over the directories themselves is External Libraries... but I don't believe uploading works with them, so you'd have to manually manage them outside of Immich (which kind of defeats the purpose IMO)

I saw a Linux program that could let you setup a virtual mount with different rules for where each folder actually physically pointed to - like symlinks but with much more customisation. You might be able to use that to setup Immich to save different users to different drives

I think they're organized into individual user directories (in one location), so could probably set up a directory backup per user to their own device if so desired.

Each user has a subdirectory, I dont see a reason you couldn't symlink them so they actually reside on different drives.

You can store uploads on a different drive if you want also.

On the phone app you go to Library -> People -> "Add a name" at the top.