From the ‘waffle’ icon in the upper right of your screen, a teacher will have access to the Student App. They may also have access to the Admin App if they have been granted any domain-level privileges.
The ability to toggle between these Apps allows a user to have multiple roles while maintaining a single user account.
If a teacher toggles to the Student App, they will see any courses in which they are enrolled as a student. Most teachers won’t have any active enrollments under the Student App.