Apply the brush where you want to erase, then click on the button (’+/-’) next to the drawn mask drop-down. That will reverse the polarity of the drawn mask, and so include everything except the mask you have drawn.
For “simple” in/exclusions you should follow @elstoc’s advice.
You can achieve just about anything you like, however complicated, by using the mask manager module in darkroom view. The part you are specifically looking for are the Set operators