The UserInfo endpoint can be used, with access token and bearer auth, to retrieve claims about the authentication event (roughly the same information exposed in the ID token).
It returns the user claims either in JSON, or as a signed JWT (see below how).
To use this endpoint, you must pass the Authorization: Bearer <ACCESS-TOKEN> header.
If the client application has any of the userinfo_signed_response_alg, userinfo_encrypted_response_alg or userinfo_encrypted_response_enc set to one of the possible values, the userinfo payload will be sent in a JWT token, which will be signed (and encrypted, depending on the referred options).