CI and CD are more workflows than tools. It doesn't really matter what your repo setup is, you just adapt your workflows to it. On one project I work on we use a monorepo for a handful of microservices. We use standard GitHub flow, no special repo consideration for the CI.

For CD, we have scripts that ask what service you want to build, and they specifically package that service using the set of files & processes dedicated to that service. The build generates a versioned artifact. After that, repo doesn't matter at all, we're just moving service artifacts around.