Can a mac boot from an external drive, if the external drive is running an older version of macOS?
A 2019 Mac is much too new to be able to boot High Sierra. In general, a Mac cannot run any macOS version whose release date is earlier than the release date of the Mac.
If you have a specific need to run an older macOS version (but no older than Lion, or at most Snow Leopard Server), you can do so in a virtual machine using software like Parallels or VMware Fusion.