How can I get ssd external hd to boot on iMac OS Monterey

If you are using a 2018+ Mac, then you need to modify the computer's security settings to allow booting from external media by using the Startup Security Utility:

About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

Make sure it is formatted APFS with disk utility before you clone backup the data with Carbon Copy Cloner or Shirt Pocket Superduper, or run the installer of the operating system on the external drive itself.