Understanding Multiple computers and Photos libraries

I have 2 computers running Mojave [as I clean out old aplibraries] and 2 newer ones, a Mini and MBP, running Mojave. Therefore, I have 4 Photos libraries - one on each. So is each a System photo library? Is it a System library per computer? So if I have photo1 on the Mini it goes up to the cloud, and Photo2 on the MBP/older it does too? And then I should on each one select iCloud Photos and also Optimize Mac Storage and select shared albums??? Just trying to figure out the best organizational structure - I assume [hope] that while I can look at both Photo1 and 2 on each separate computer the 'master' original is up in the cloud???