Thanks for your feedback!
You landed on https://join-lemmy.org/docs/users/01-getting-started.html to get started but you should have landed on https://join-lemmy.org/ which is a much simpler UI, i think. Somebody sent you the wrong link. I think, there should definitely be a more prominent link to the actual "sign up here" page.
I’d separate the IdP (identity provider) from the service/content providers
That is indeed a good idea and i've never heard it formulated like this before, but i gotta think about it now.
After that you are told that your registration needs to be approved manually and that there is no notification about that so you have to manually check from time to time whether your are able to login or not.
That is definitely a big problem that should be changed; I'm not sure but maybe you should get an E-Mail after your registration is successful and maybe you should also be able to log in and use the account immediately (up to a limited extent) without waiting for manual confirmation.