Any single piece, on land or sea, blocks all land or sea combat movement in the way you contemplate. The only exception is air flying over it.
this is mostly true but there are a few exceptions:
as he says, all planes can fly over anything except neutrals that aren’t violated yet.
boats can ignore transports and subs and move right through them or end movement in the same sea zone as them. one restriction is that transports can’t load or unload in a sea zone with a a sub unless there is a surface warship along with it.
subs can ignore (move through) all ships except destroyers. they can also end their movement in sea zones containing any ships on non-combat.
I think those are the only exceptions for units blocking movement or being able to move through territories. If I forgot anything, one of the better players than me will notice it :-D