How is it like studying a Masters in Germany?
I studied in Germany, but a different master. However, giving a general answer to your question is very difficult, especially whether the professors are helpful.


You may end up in courses which are so large that the professor might only know a couple of students by name. 
There are also other courses with only a few students where you are in closer interaction with your professor.
Please note that there may be a large number of courses which are taught not by the professor himself/herself but by employees of his chair, e.g., by PhD students.


I had courses from all those three categories listed above. This does not only depend on the university (some might have smaller groups than others), but also on the courses you decide to take.

It is accepted and encouraged to ask questions during or after the lecture. 
However, interaction with your professor may not be that important in total and you should also be able to solve most issues on your own.

Even for your master's thesis this may apply: Some professors want to be involved in their student's work but I have seen different behavior: For example, the first and last time I saw the professor during my bachelor's thesis was during the final presentation. The work was supervised by an assistant and he was very good, so I was OK with this!