git subtree workflow

I'm fairly new to git and am trying to determine if git subtree would be helpful for managing our company's codebase, which has several repos some of which depend on each other. All the examples I've seen make sense to me as a one-time operation to merge separate repos into one monolithic one or to split one monolithic repo into separate repos. I'm having a harder time understanding how this fits into a larger picture and what the workflow for working with subtree would be.

If I have a bunch of repos on GitHub and some depend on each other, how would I set them up to work with subtree? Would GitHub continue to host them as is, host a merged monolithic repo, or host both a monolithic repo and the splitted out repos? The exact answer probably varies, but I imagine there's basic workflow that would satisfy 80% of users. If I have GitHub host both monolithic and splitted out repos, it seems unclear as to how I keep those repos in sync and make sure all the developers in our company push their changes to both repos.

Thanks, Ben