What kind of questions are asked by the adviser?
What sort of questions do you ask your advisor during an online interview?
I have experience of this on both sides.
As a supervisor, I want to determine whether the student has qualities that will allow him to succeed in the project and contribute to my research. In particular, I am interested to know how successful he has been in his studies, as it a sign of self-discipline, and I ask detailed questions about skills needed for the particular project (e.g., programming skills). I also pay attention to how easy it is to communicate with the student, e.g., whether he can precisely understand questions, answer them directly and in a clear way, etc. I also want to know his motivation and whether he wants to pursue an academic career or just get the degree and then search for a job outside science. Those who just want to get the degree are likely to make minimal effort during their project (just enough to get the degree), so I prefer those who want to pursue an academic career. Expecting this, students almost always say they intend to work in science, and then I ask why. There are also many other things.
As a student, I wanted to know about the project and, in particular, how helpful the project would be for my career, what I would learn by doing the project, how significant the project was, etc. There are also many important factors that you find out by doing your homework: the academic status of the prospective supervisor, his publications, citations, publications of his past students, careers of his past students, etc.
Also, Which one do you choose during the interview; turn the webcam on or off?
My advice is to turn it on, even if the supervisor doesn't. In this way you are more open, better show your reaction, create a better impression, etc.
If the interview goes well, and the advisor asks you when you can you join the group, but you have other interviews coming up (or you are waiting for them), how do you answer his/her question?
You can signal that you like everything and that you need some time to read more about the topic and take a final decision. You are not expected to take your decision instantly. It is a serious matter.