Earlier quoted context omitted.
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. https://github.com/immich-app/immich/discussions/14365
There’s no download all in apple photos either unless they added it
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.