Given that you said "learn about" and not "use", and given that you said that you had MacOS, I'd say learn FreeBSD first, then NetBSD, then OpenBSD.
It's worth learning them all.
But #FreeBSD is the closest to what you have in MacOS, with there being somewhat of a common heritage from many years ago; #NetBSD will expand your horizons from that, and then #OpenBSD is the furthest away from MacOS with very different ideas about almost everything and different abstractions.