how to boot my macbook pro from a external drive

If your Mac has a T2 security chip, you first need to discuss this with:

About Startup Security Utility - Apple Support

Just about any external drive with the correct interface will work fine, BUT: Do NOT use the maker's drive Utilities, ERASE the Physical device before first use with only Disk Utility.

I've done this, following the advice from "About Startup Security Utility). The problem, as I said above, is that the desired volume does not show up as an option. All the other connected volumes do show up, just not the one I need, the bootable clone created by SuperDuper!. Other volumes on that drive show up, so the Mac is seeing the hard drive, just not the volume I need.

OS Monterey 12.0.1 specifically WON'T install on 3rd party SSDs, even internal ones...

Hopefully Apple will fix this asap - it's very inconvenient, and an issue that isn't widely known about yet.