What is a user (Benutzer/-in) and what is a user account (Benutzer-Account)?

A user is a natural person who has a user account at a relying party; the user account holds the user's attributes and data at that relying party and is linked to one or more digital identities that can be used to authenticate towards it.

The account is created when the subject registers at the RP. Keeping "account" (the RP's record) apart from "digital identity" (what you authenticate with) explains social login: one account at the shop, linked to a Google identity and an Apple identity. Synonyms: user object, Benutzerkonto.

Go deeper:

• User (computing) (Wikipedia) — users, usernames and user accounts across operating systems and services.