Your reverse-proxy auth-service!
SSA is something like Authelia, but (in my opinion) simpler to set up. SSA needs a reverse proxy to run.
tl;dr
Got a reverse proxy and want to build SSO into it? Then SSA is the right thing for you.
You want more features? Have a look at authentik or Authelia.
Have a look at the docs.
[^1]: The backend is written with Rust