You can stay in Germany for longer than ninety days if you have a German residency permit. You are free to work, study, or do other things in the nation during that time.
However, how can you tell if you require a residency permit? For stays longer than ninety days in Germany, foreign nationals other than those from the EU, Switzerland, Norway, Iceland, and Lichtenstein require a residence permit.
To enter Germany for 90 days during a 180-day period, many foreign nationals will need to apply for a resident permit from their home countries or obtain a Schengen visa. However, for those initial 90 days and for some nations, it is allowed to enter Germany without a visa.