Pathfinder was to get around WotC dropping D&D 3.5. Paizo was started by veteran D&D writers to sell adventures, which they still do as adventure paths, rather than a system. When WotC updated to 4e, meaning no more print books that Paizo could reference in their adventures, Pathfinder was a way to print new 3.5e PHBs and Monster Manuals.

Paizo didn’t initially change much in PF1e. There were a few balance tweaks. The books were better laid out than 3.5. The players did the math on things like combat maneuvers in advance. In practice the game played pretty much the same, my groups jumped over seamlessly.

Having run and played both, I do think Pathfinder 2e is counterintuitively simpler in play than 5e D&D. 5e plays fluidly almost immediately, move and act. PF2e is pretty demanding for the first hour or three, the three action economy and Conditions (tm) are an armful, and many players need to unlearn some D&D habits. Once a player has below average system mastery PF2e is as fluid as 5e. Beyond that PF2e shines. The rules scale better to complex scenarios, giving players more clear options of how they could act and giving the GM a better framework to figure out exactly what someone needs to roll. I also think it’s easier for players to go from average to good system mastery in Pathfinder, it’s mostly just learning how to optimize their character and learning more conditions and spells that work in the framework the player already understands.

For new players in session 1 D&D is simpler, in session 5 Pathfinder pulls even or maybe ahead, and in session 50 Pathfinder still sort of works where D&D falls apart.

PF2e character customization, though, is much more complicated, which some people like and others do not.

Eh, yes and no.

Pathfinder 1e was pretty much just straight-up continuing D&D 3.5e, but with some tweaks. Pathfinder 2e overhauled a lot of stuff, often simplifying things, but still pretty complex.

Compared to D&D 5e, Pathfinder has more rules, but those rules often make things easier, or (IMO) get you more return for the effort. So, for example: The feat list is bigger and more complicated, but in practice, it means you only need to look at a handful of them when you level up, which is easier (and the rules give you guidelines for swapping things out if you don't like them). The monk has more decisions to make with stances and attack types, but that's... kind of what you want with a monk to make combat interesting. There are rules for boats, and holy shit how does 5e not have rules for boats.

The last example might sound silly, but it's part of what convinced me to switch. It's an annoying omission in and of itself, but also speaks to a broader pattern of 5e just not supporting Dungeon Masters, letting them fix the either broken or incomplete rules, or else take the blame for them. Pathfinder actually supports ~~Dungeon~~ Game Masters, as though their time, effort, and fun were just as valuable as anyone else's. /rant

Pathfinder 2e is what I'd play if I wanted something like 5e, but runs differently. If I wanted something similar, I'd pick something else, but that's a longer, even more off topic discussion to go into unprompted. :P

Pathfinder was created as an updated version of D&D 3.5, which was very complex. PF food streamline parts of it, but ended up just as complex at some point, mostly due to the massive variety of options available through splat books.

Meanwhile, D&D 5e was released to be much less complex by getting rid of stacking bonuses and the vast majority of math.

Parhfinder 2 (which I have not actually played yet) did not do that. They opted for streamlining the existing system by combining several similar subsystems into one (i.e. everything is a feat now). But the math is still there.

Yes it is. Pathfinder made for builders who want to create a character with hundreds of options to choose from. It is rule heavy in the tradition of dnd 3rd edition. Pathfinder 2e is much more refined, but I doubt they went away from this philosophy. It's still very rule heavy.