os 10.12 no longer sees a TB drive that has been loaded with os 12.0.1

That's correct. 10.12 is too old to see Apple's APFS format. Any solid state drive will automatically format as APFS on 10.13 or later, and 10.14 or later can't install 10.14 or later on solid state drives unless they are formatted APFS. 10.13 is the earliest Mac OS supported APFS. 10.2.3 through 10.5.8 only recognize Apple Partition Map HFS Extended Journalled. And 10.6 or later recognizes GUID on Intel Macs as well.

If you need to boot off an older drive to see a new Mac, consider using file sharing alternatives over a network.

AnyDesk has older versions, Teamviewer has older versions, and Apple includes in

System -> Library -> CoreServices -> Applications -> Screen Sharing for accessing Macs that have screen sharing turned on.

https://mbsdirect.com/mbs-blog/article-use-the-macrsquos-built-in-screen-sharing-to-provide-remote-help

The nice thing about screen sharing is the Mac that runs it can drag files out of the screen sharing window onto its folders, or vice versa enabling file transfers as seemlessly as on the Mac file system itself.