I've seen many professional guitar players use massive rack setups, usually comprising several single-effect rackmount units. Yet they somehow managed to control all these effects from one single (usually quite large) pedal. How is this achieved? What are these large pedals called? Any information is appreciated.
They're connected to switchers typically, like a GCX or similar unit, which have all the midi units connected through midi cables and they usually run in line like a pedal or in the switchers loops.

Switch the units/pedals you have within it.

But you dont have to have all your rack units in the loops, you can run them all in line if you want, and put pedals stored in the rack in the loops.