It will always eat a bit of it, even sane muscle, the process is not perfect. But old cells and sick cells are prioritized. That's one reason it has benefits in combination with chemo therapy : the body will target the cancer cells. Not only those, but it's enough to help.

Same with weak joints, old muscle bruises, etc.

But again, eating properly after the fast is important to let the body rebuild what it destroyed.

what about brain cells?

I haven't read any study on brain cells specifically. My guess would be that the mechanism is the same and is not locally different according to the different body parts but i can't be sure.