I would like to ask, if somebody knows, why there are no roles within the user details in REST ADMIN API request. I saw some posts dealing with this topic, but
You are not getting roles in the user details because the REST API is strictly resource based and roles are separate objects that are just associated to a user. The following REST URLs can be used to get a user's roles
Getting the associated realm roles:
GET /auth/admin/realms/{realm}/users/{user-uuid}/role-mappings/realm
Getting the associated role of a specific client:
GET /auth/admin/realms/{realm}/users/{user-uuid}/role-mappings/clients/{client-uuid}