Stephen Hackett:

It used to be that to run a Time Machine Server, you needed to be running a copy of macOS Server on your host machine, but those days are now gone. Anyone running High Sierra or later on a Mac can now turn that Mac into a destination for remote machines to use for Time Machine.

Say you have a Mac mini on your network, and a MacBook Pro. You can hook up an USB hard drive to that Mac mini, and within a few minutes, be backing your MacBook Pro up across your network using Time Machine.

Saved for future reference, when I eventually retire our Time Capsule and buy an Eero kit.