Compleatly forgot to answer your other quesntion:
Yes the main income is donations. The difference is that you don't need to pay if you can't afford it and people who donate are generally paying more on average. The huge difference is that everyone has access to everything without paying.
Imagine what would happen if you would have to pay to use Wikipedia: nobody would use it. The content is made by users and if they would have made it a paid encyclopedia it wouldn't have any users.