Chosen Solution
So basically i have a MacBook Pro 2014 that I replaced the SSD from a MacBook Air of the same year. I tried to install macOS Catalina on it (as a HFS+ drive) and it installed and then the MacBook doesn’t detect the drive to boot off of. Is there any way to fix it? (I can reproduce this when I try to install OS X Yosemite on the drive as well) the OS detects the SSD. the boot menu will not.
Mojave and onwards will only install APFS on the boot drive. HFS+ will not support Mojave or Catalina. In addition the systems firmware needs to be updated to support APFS as well, if the system was not properly upgraded to the needed firmware you can find a drive which you expect to work won’t! So to properly set this up you need to use the OS recovery services to reformat and install the older macOS to then upgrade the different OS versions to Catalina Vs jumping directly to an SSD which was setup off another system.
