You don't fly many aeroplanes at Vmd or Mmd, and for the same reason you don't necessarily cruise below Mcrit.
Min drag is about best endurance speed. Aeroplanes don't usually fly around that, as the real objective is to get to the destination with the minimum fuel burn. That is best done at the higher best range speed.
Real world economics however push the cruise speed up further, as there's more to the cost of running an aeroplane that fuel burn, and the airline will want to maximise the utilisation of the aeroplane (and arrive on time!). So, the aeroplane will be flown at higher speeds again. Hence flight above Mcrit.
As you approach transonic speeds, there's a pitch trim change. A Mach trimmer links into the pitch control circuit and applies what in effect is a pull back on the stick of the right amount at the right Mach number(s) so that the pilot doesn't have to. There's nothing particularly complicated about it to be honest, and it's really just there as a safety improver / workload reducer. The internal mechanism is really just mechanical gearing (or electrical / electronic simulation of the same).
And the plural of "aircraft" is "aircraft" not "aircrafts". A common mistake in non native English speakers.