It looks like there must be system calls to create and manage APFS snapshots on external drives. Carbon Copy Cloner uses them in their backup routines https://bombich.com/kb/ccc5/leveraging-snapshots-on-apfs-volumes
But, there doesn't seem to be a command line interface aside from
tmutil can view snapshots by volume, but can only make snapshots on the system volume.
There must be an API call, or procedure, to create an APFS snapshot, that CCC uses?
I have some familiarity with APFS and lack of command line tools, as I use AFSCTool a lot (https://github.com/jeabraham/afsctool/). Since Apple doesn't provide a file system compression command, someone needed to write a C program to call the APIs to compress. (Apple supports on-the-fly decompression, but doesn't give users a way to compress the files in the first place, so someone else had to write a command line tool for us.)