It takes a while to figure out how selections in Gimp work.

It whenever you select you have created a mask and when you combine it with layers it can get very confusing.

If you accidentally select a small bit you cannot edit anything else. I think that is what OP is referring to.

There is a tool that shows you what you have selected that can help.

IMO Gimp isn’t very well documented so you can get stuck for a while before you understand what is going on.