Authelia is an authentification provider. So you can have a single login for all your services. It can provide autorisation and authentification with a single unified login.
Bitwarden is much "simpler", in it's just a passwordmanager. As soon as you start sharing passwords, like you do in bitwarden, you lose the authentification part, even worse, you lose control over the shared login. Anyone with autorisation can "steal" the login as in unauthorized copying/distributing the password or even changing the password alltogether.
With an sso like authelia you can mitigate such attack vectors.
I see a big problem in every approach, probably because I don't understand something
When i'm using just bitwarden, all my passwords for every service are different, but the ui is opened for anyone to see
When I use authelia without oidc I add complexity of using the services, and probably two passwords to type manually, or a locked down system(which is cool)
And if I use authelia with oidc, it means I have only one password for all of the services (manual, or in bitwarden (which has its own manual password))