This is merely one of many questions you will end up learning the answer to if you pursue this program.

My guess is that you only have to compile the build target that your code change was part of... you can build just Chrome, or just unit tests, or whatever else is part of the package...

Also a common way to speed up build times is to build in release mode.

No. That's why make(1) exists. You do usually have to compile everything once though.