When you join a course, you are automatically enrolled in the corresponding course/group by the course administration.
The Nbgrader extension in JupyterLab has a graphical interface that can largely automate the process of (weekly) assignments and submissions.
As a student, click on Nbgrader → Assignment List in the top menu:
If you are enrolled in more than one course, select the correct course:
The upcoming assignments will now appear in the list and can be copied to the local environment using the button:
By clicking on the relevant assignment, you can now edit, validate, and submit it back to the teacher:
You can also validate your answers in the editor. There is a button for this in the Editor toolbar: