Pathfinder 1st Edition was a branch of DnD 3.5, it is occasionally called 3.5.5 or 3.75. It is pretty much a 3rd party patch for 3.5 but uses the same core systems. That, 3.5 and PF1e, is kind of a mess.

I'm not surprised you find PF2e confusing, but from a design standpoint I would call it clean, considering everything that is going on. It is deep, but well organized. As opposed to DnD 5e, which is relatively shallow, which can make it easier to jump into, but not as well organized. The messiest part of 5e is the "natural language" philosophy they went with, which can leave a lot of rules ambiguous. It was supposed to make it easier to intuitively pick up and play, but it also makes it much easier to have misconceptions and anything that is slightly unintuitive can easily be accidentally used the wrong way for ages. PF2e might have a lot of interconnected rules depth, but it also has a less ambiguous guide for dealing with it, which is what enfranchised players will generally mean by "clean."