Not a Googler, but I think the answer is: yes. At least, it is for my monorepo company.

Usually somebody else has already gone through the work of doing it for you. Sometimes there are tools that do the translation for you. For example, Go modules are quite easy to translate to a BUILD file.

It’s actually not as bad as it sounds. You only have to do the hard stuff once, and every engineer in the org who uses it in the future is thankful for it.

They use a tool called Blaze (Google around for “Bazel” which is the open source tool inspired by it). Basically you model the dependency tree such that the tool knows which targets are affected by a certain change, and then Blaze builds them in a clean room environment such that an undeclared dependency would cause the build to fail (hermetic builds). As far as I’m aware, this is the only way to sustainable operate a monorepo, but I would be happy to learn more if someone has other solutions.

I assume you mean third party dependencies that are not in the monorepo? Pretty much yes, monorepos struggle if they are expected to handle dependencies that aren't stored in the monorepo, so step 1 of using a dependency from outside of a monorepo should be to copy the source into the monorepo (and transitively copy the source of dependencies, etc).

Full dependency tree yep. No build in google's main repo ever retrieves code externally.