Currently when you open another app which has a username/password inputs you get the option to “fill from vault”, but this only lists items which match the current app - you cannot choose another vault item to fill with.
(e.g. sign up for “example.com” on a browser and save to vault, download the “example” app, get app’s login page, tap Bitwarden’s “auto-fill from vault” - you cannot search for the existing vault item you have for example.com)