When a user has been assigned to a product license for Finance and Operations, a user will be automatically created the first time he/she logs in. This is as described in the documentation:
"For customers who are on one of the new license types that were added in October 2019, users must be associated with a license. Users who are associated with a license are automatically added as system users who have no roles the first time that they sign in."

The user ID has a pattern like '$BC8A'. It is a dollar sign with 4 hexadecimal values; which is meaningless and not user friendly when user IDs are presented on forms.
It would be better to have a meaningful user ID. Probably allow a configuration or just take the local-part of an e-mail address. In case of chrisg@contoso.com, the user ID can then be 'chrisg'.