Why would you test dependencies? Test the code that you write. Do not test the code that you did not write. Literally, you should not test dependencies.

I never got caught into the monorepo fad, I'm glad I didn't, and I still don't see a point in having monorepos. Let's say in ruby on rails, I have the host app and 3 mounted gems in it. That's 4 repos. It could be all merged into one monstrosity but separate repos help keep complexity down, and help define interdependencies cleaner and simpler.

The monorepo idea was just a fad. Each package should be its own repo. Each package should be meaningfully useful, e.g. one package for string utils, not one package for trim, one package for pad, etc.