Most Google engineers don't do anything more complicated than the hello world examples. Probably far less complicated because they don't need to do any repo setup, it all just works out of the box. Most engineers just make simple java build targets. Larger teams would have someone who specializes in tweaking the build and writes Skylark rules, but to other engineers they just look like simple build targets.
I'd suggest finding an open source project and playing with it locally. Try to get everything building and running, including unit tests. Make the build as granular as you can. Try to pick a project that has a variety of kinds of things that need to be built, like Java, C++, Protobufs, etc. Then look for patterns in the build targets you wrote and see if you can refactor them to simplify it with custom Skylark rules.
> but to other engineers they just look like simple build targets.
I forgot to mention, this is by design. It is uncommon at Google for a BUILD file to be more than 3 or 4 build targets. Maybe someone will add a for loop to the build so that they can define various flavors of the build easily, but that's it. Anything more complicated and it should be extracted to a separate Skylark file, included, and in the BUILD file it should look like a basic task.
I'd suggest finding an open source project and playing with it locally. Try to get everything building and running, including unit tests. Make the build as granular as you can. Try to pick a project that has a variety of kinds of things that need to be built, like Java, C++, Protobufs, etc. Then look for patterns in the build targets you wrote and see if you can refactor them to simplify it with custom Skylark rules.