Is it a study visa or a work visa to study for a doctorate in Germany?

If you are a university professor and hold a scholarship, it is usually a study visa; However, if the other party is a German research institute, even if you hold a scholarship or a public school, you generally need to apply for a work visa, because it is not an admission notice from a university, but a letter inviting past research.

If it is a doctoral position that needs to be taxed, it is obviously half a job. Even in a university, it should be a work visa.