Representing the pure perfection of order, the pinnacle of lawful behavior, Mechanus appears as an infinite array of clockwork machinery, stretching into the horizon. In this vast plane-machine, life clings to gears the size of continents and pistons the size of planets, completely in tune with the ticking of the multiversial clock.
Mechanus' most iconic inhabitants are the Modrons, geometrically shaped extraplanar beings fashioned of otherworldly machinery. Come third edition, modrons lost their place of prominence and were supplanted by the Formians and the Inevitables, something explained officially as being due to the death of Primus at the hands of Tenebrous (the undead Orcus), as detailed in the adventure "The Great Modron March". Unofficially, it's generally held that modrons were pushed out of the limelight because they were seen as embarrassingly goofy by Wizards of the Coast. Fortunately, like other goofy old-school things, they were rescued by 5e, restoring them to their full, somewhat silly, glory, cartoony-as-fuck art and all.