How easy is it to reinstall MacOS and change my disk setup? I have a 2012 MacMini running Mojave. Originally MacOS was on the built-in 1Tb HDD but I got an external SSD and set this up as the main drive for OS, applications, etc. Given the Mac will no longer boot without the external drive, I believe I must have removed the original OS from the internal drive.

I would like to revert to booting from the internal drive, in a way that causes as little disruption as possible. Ideally I would be able to move installed apps rather than lose them, and certainly not have to format the drive. The external drive would be removed or become just a data drive.

How can this be achieved?
Install to the internal using erase install steps. 
Migrate data / apps / settings from the external using the migration assistant portion of the os setup assistant.