Right now none of the native clients support SSO. It is a frequently requested feature but, unfortunately, it doesn’t look like it will be implemented any time soon. As with many OSS projects it is probably a case of “you want it, you build it” - but nobody has actually stepped up.

For web access, stick it behind a reverse proxy and use something like Authentik/Authelia/SSO provider of your choice to secure it.

For full access including native clients, set up a VPN.

Immich has completely replaced Google Photos for me, love it!

My only bugbear is that it is updated very frequently (what a nice problem to have!) which in my case requires a manual once-over of my docker-compose file every time in case there are breaking changes.