In my limited-but-not-nothing experience working with mono vs multi repo of the same projects, CI/CD definitely was one of the harder pieces to solve. Its highly dependent on your frameworks and CI provider on just how straightforward it is going to be, and most of them are "not very straightforward".

The basic way most work is to run full CI on every change. This quickly becomes a huge speedbump to deployment velocity until a solution for "only run what is affected" is found.