Much of what's happening is less like 'calculations in the back of their head' and more like 'pattern recognition'. Chess masters are recalling ideas and motifs (not necessarily actual positions) from earlier games or practice or study, and this recall is allowing them to either prune the decision tree or prioritize investigating more fruitful branches faster than amateur players, who have to actually calculate things that are instantly 'seen' by the better players.

This is a known thing among chess players, and even low-skilled players do it to some extent. A newbie to chess can (and almost every teacher will say 'should') work their pattern recognition by just solving tactical chess puzzles and very quickly improve at spotting things like one or two move checkmate patterns or forks or pins or whatever.

It's more of a memory thing than subconscious calculation.