If you mean how they're moved in the sense of how does Cache make sure data is durable in the database and consistent even if there's a crash, you might look at the data integrity guide's section on the write image journal (WIJ):

https://cedocs.intersystems.com/ens201814/csp/docbook/Doc.View.cls?KEY=G...

You might mean a number of other higher-level things though.

Async mirrors are commonly used for reporting and other purposes.   What do you want to scale down to use it here?   

Mirroring is about creating identical globals on multiple machines with one update.  I assume you don't want these systems exactly the same, but depending on what you do want, maybe you could find a configuration which would do it.  For example, maybe you could put the global you want replicated in its own database on an async member and not mirror any other databases.