It’s the topology of the object. Essentially flatten the object into a 2 dimensional plane. Something like a mug or donut will have a hole in it after flattening, these items have 1 hole. Different objects flatten into a different number of holes. It’s really hard for me to parae anything more complicated than 1 hole with my head, but that’s essentially the process.

I do not recall the vsauce video explicitly, but this should be the logic they’re using. It gets waaay more difficult to parae on something like a human, with a bunch of different tracts and splits and shit.