As far as I know, with a valid residence certificate, you can directly invest in opening a shop as a boss, and you can get a boss visa for one year, but there is a limit on the investment bottom line. In Germany, the registered capital is 3.5111 euros. In Spain, you can directly change from a student visa to a boss visa.

You can't work in a country without a work visa if you have a work visa from another country. If you want to work, you must have a boss to apply for a work visa before you can work. The specific procedures and fees should be asked by yourself.