Beware that migrating back from Immich to iCloud/Google is not something Immich cares about. There is no "download all" anywhere, best way is to go to the server and get raw files from there.
Backups are easy too: you can very reasonably just rsync the library folder, and it'll be recoverable from that alone (I tried it! Worked great). You'll lose accounts and image-content search and iirc faces (possibly painful), but most of that is trivially rebuilt or not very interesting for single-person scenarios.
If you set the pref to keep originals locally, they're all on your drive, in original form, as well as the derived versions including caches of raw to jpeg, resolutions, and edited versions.
That said, Apple Photos does let you export even if only in cloud. Open the library, select all, and File > Export ... > Export Unmodified Originals.
It pauses for a second or two on my quarter million images, but is then happy to comply.
https://privacy.apple.com/account -> "Get a copy of your data"
Then you will need to chose the maximum size of an archive part and wait for link to a download page.