,Creating a bootable clone

I want to create a bootable clone of an old (Intel) Macbook Pro running Monterey that I can use to boot my new (Apple) MBP running Ventura (because it turns out Ventura doesn’t support my Time Capsule backups.!)

I got a new external SSD and erased it with Disk Utility settings MacOS Extended (Journaled) and GUID Partition Map. I then cloned the internal SSD to it using Carbon Copy Cloner - I would have use Disk Utility but it said my 1Tb SSD was too small to clone a 700Gb disk. That all completed successfully but when I try to boot from the external drive it is not listed as a boot option.

What am I missing?