I've noticed while building in the Nether, that Zombified Piglins that spawn in my base always want to be as high up as possible? I encounter none of them spawning in the lower section, while preferring the upper platform. I just built a staircase leading up into an even higher area, and immediately multiple ZP's began to pathfind up, and continue to pathfind there, even though I kept nudging them off the edge. Is this regular for Zombified Piglin AI? Do all mobs do this and I just haven't noticed?
Nope, this is just normal Minecraft pathfinding logic. Each mob will search a 20x by 20z by 15y cube centered on their current position, randomly choosing an appropriate block to pathfind to. This spot must either be a non-transparent solid block (dirt, stone, wood, etc) OR a transparent block (glass, stairs, slabs, fences, etc) with a non-transparent solid block underneath. If the mob cannot physically reach the destination, they will move to the closest spot to it.
Gnembon has a fun farms video about a redstoneless blaze grinder that uses pathfinding to control where mobs go, I believe it's Fun Farms #9 (with Fun Farms #11 revisiting the blaze grinder to make it even simpler). He explains everything in detail there.