there are similar border crossings between Norway and Sweden. There, the problem is contraband of drinks and fags. Each lorry goes thrrough an x-ray machine and the customs officers will decide which ones to pull in for further checks.
The operation at Dover will of course change after the transition period but in itself not much different from Felixstowe and London Gateway Port which handle a similar amount of freight, a lot of it from and to outside the EU, without the 17-mile queue.
I used to drive up to Heathrow T4 to collect shipments of mainboards and video cards from Taiwan. Honestly, the volume of freight at Heathrow was already mind boggling 30 years ago.