Both Swiss and McMahon are pairing systems; they’re used to decide who plays against whom. Basically, players who are close in points are matched up against each other.
In Swiss pairing, it’s possible for the strongest players to beat on the weakest players resulting in multiple people getting perfect records, resulting in tie breaks determining who wins. McMahon was designed to avoid that situation.
The idea with McMahon is to help expedite the process of actually finding a winner. Basically, the higher ranked players are expected to win, so they’re given extra points and start off playing against each other earlier on in the tournament (because players who are close in points are paired with each other).
I don’t think non-simultaneous round robin tournaments are possible on OGS.