Am by no means a concrete expert, but learnt a little by osmosis from family over the years. Like most civil engineers, these guys had an unhealthy and rather weird affection for the sloppy mixture of gravel and sticky stuff. And loved to build things with it. Heavy things, actually very f…ing heavy, whereas my comfort zone is lightweight aluminium structures.
Anyway, the trick with mixer trucks, as I understand it, is to keep mixing until ready to stand and deliver. Apparently the gooey, wet stuff has to stop moving and be allowed to sit perfectly still, in order for it to set and go hard. So keep mixing and it cannot set. That I believe is what mixer trucks do - they have an auxiliary drive from the main engine, which rotates the drum and keeps it rotating, right up to when they're ready to pour, at the job site.
Once the pour is done, the drum and all its connections, pipes and fittings etc are washed by copious amounts of water.