This is no different from the situation in Linux, BSD etc, where executables and libraries are simply mapped into the process with mmap() - they're backed by the files and the system is free to discard those clean memory pages as it sees fit.

When you install(1) a new version of a file, the old one is deleted and the new one written in its place. The file handle in existing processes continues to point to the original, and the filesystem will wait until everything's closed it before actually discarding the file data.