I am going to obtain masters degree in mathematics in Berlin. The market in this field is extremely competitive, so I think about "Plan B".
How long can I stay in Germany after graduation? (I heard that 3 years, is it true?)
Can I work in different area? I mean not as a mathematician: I can work as programmer in IT or, if I will not be lucky, as unskilled worker, like courier, carrier, porter, waiter, stuart,...
Who or what decides what job is appropriate for my degree?
If I, for example, find such unskilled/"differentskilled" job during or after graduation, can I stay in Germany?