good practices to modify tensorflow sources

I'm trying to modify Tensorflow source code (core).

After study the code I started to modify, recompile and run my own tests. But I'm not sure if I'm doing it in the best way possible. So I have some questions about it.

1 - Do I need to recompile all the tensorflow code or only the modified module?

2 - To implement the modification, have I always to create a wheel package from my sources and install it? Why I can integrate the modification directly after the bazel compilation?

3 - What is the best way to test the code? There is a lot of units test and I don't know how use its properly.

4 - I'm using bash scripts for linux to run bazel commands, there is something better?

Thanks everybody!