Register for your stay through the REST-Directive
Within the framework of EU mobility, you can study in other member states (except Ireland and Denmark) for up to 360 days with a residence permit for study purposes. The Research and Studies (REST) Directive did not apply to the United Kingdom even before its exit from the EU. Denmark and Ireland have not implemented the REST Directive.
If you already have a residence permit for study purposes from another member state, you do not need a German residence permit. Your residence permit must be valid for the entire duration of your stay in Germany.
The German university must notify the Federal Office for Migration and Refugees of your stay in advance. Your personal appearance at the immigration office is not necessary.