University Job and Dissertation (Ph.D.) Work
To understand the German Ph.D. system, it is helpful to understand the difference between a university job and doctoral (dissertation) work. Many confuse the two; they are orthogonal (but do interact).
A university job is just that: a job. You are employed by the university for a decent salary, have a manager (typically a professor or a substitute) and are put to work on a project. Helping with teaching is also usually part of the equation. The job is bound by the usual rules and regulations: defined working hours, social benefits, salary. There are no implications on getting accepted as a Ph.D. student and performing research work and writing a dissertation.
Doctoral research work, that is the research work leading to a dissertation, as well as getting approved as a Ph.D. candidate and submitting and defending a dissertation is independent of a job at the university. For example, I have several external “Ph.D. students” who are self-sponsored, which is to say they have a regular industry job that pays their bills and they are trying to do their research work on the side.
In Germany, there is no Ph.D. program, really, so the term Ph.D. student is actually a misnomer. People who work on a dissertation are researchers guided (supervised) by a professor. There are various measures of helping such people become better researchers, but it is not like a Ph.D. program at Anglo-Saxon universities that you can enter after a Bachelor’s degree. That’s why in Germany you can be awarded official Ph.D. candidate status only after you finished a Master’s degree.
The tie-in between university job and doctoral work is that the project you are working on as part of your university job typically has significant research substance. That substance then can be used to write the dissertation. If you are lucky, substantial parts of the project work can directly feed into your dissertation. There is no implication, however, that a university job gives you extra time for doctoral research work if it does not align with the project. Again: university job and doctoral work are independent of each other.
However, the interests of a professor and the research employee (internal “Ph.D. students”) are closely aligned: The professor wants to see research papers getting published, and the research employee wants to publish papers as validation of the work they are putting into their dissertation. Thus, a good professor will try to allocate some of the project work time from the university job to writing research papers based on that work. This is the opening for the employee to write the dissertation.