Transferring Catalina system drive to older Mac — possible?
In the past, you could quite simply move a boot drive from one Mac to another &amp; have it work as though nothing had changed. The only proviso was that both machines were of an age to know what each OS is, not too new, not too old.
I've been doing this with Macs for decades. 

If the 'jump' between OSes would have required a firmware update, that may be a stumbling block, however, if the Mac you want to transfer the drive to cannot run the OS on the drive, then this simply will not work at all.