Simplicity is the reduction of complexity, at least on the outside.
Apple products appear to be minimalist but the guts of an iPad or iPhone are full of strategically placed microchips and other delicate engineering.
Simplicity requires strict reduction too. Deleting the unnecessary or less important is like a game of chess; each move is a guess into how the wider game will unfold.
Simplicity is the end-product of all types of work and problem solving done behind the scenes. As a user, the final build appears to be so easy.
Hiding complexity while providing top usability is therefore the greatest challenge in attaining simplicity.