Good luck finding anything that still seems "simple" when you really look towards understanding how it works rather than what it results in. Simple and complex always have to be relative to other things as nothing is absolutely "simple" e.g. even a single atom is an extraordinarily complicated environment which spans many textbooks and degrees worth of interactions and understandings. To say everything is just complex to more super duper extra complex is to just replace the sound of "simple" with the sound of "complex" yet have it mean the exact same thing and use it in the exact same way.