Computers used in Computer-Based Examinations?

So I just gave a computer-based examination. I don't know how it is in other countries but I have given 2-3 computer-based online exams here in India, and I have noticed that in these computers, the browser window(I can only assume it is a browser, it may be some other program) is like fixed on the screen in fullscreen, I can't see any close/minimize buttons or anything, the entire screen is covered by this program and the window can't be closed or minimized. I am just curious how this is possible. Do these computers have some special Operating System installed or is there a way to do this in Windows(or linux)? If so, how?