Authelia is meant to be an SSO (like Google). In order to use it, you have to create users (and passwords) within the authelia yaml file, or connect it to light-ldap and do it via ldaps web gui.
You probably have other services running, i.e. immich, etc. These can be configured to use auhelias OIDC to authenticate the user against. you’d still need to create the users within the service, since I doubt they get auto-created.
Now, you can decide for yourself, whether to put your bitwarden behind authelia or not, and I’m not sure how the mobile apps work in this sense, if at all.
If you decide to do so, you just give your users their authelia/lightldap creds, if not, you additionally have to give them their bitwarden creds.