Maybe (probably) I'm missing something very obvious, but I couldn't find in the Rulebook what happens to your HP while you are transformed with things like Polymorph or Animal Form. I know in 5e your HP is completely overwritten, so if you transform into a beast that has 100 HP and you take damage or get dropped to 0 during this form, nothing happens to your original self. Actually that's why Druids have basically infinite HP at higher levels in 5e.
How is this handled currently in the Playtest, is it the same as 5e, 3.5, PF1 or a completely different way?
First problem, when you polymorph your HP doesn't change. You get exactly the changes listed in the spell, nothing more or less. You don't look at the creatures in the bestiary at all. I don't know 5th very well, but it sounds like PF2 and DD5 are radically different in how they work polymorph spells.
The funny thing about the polymorph spells in PF2 is that they don't actually change your stats. They change your form, they give you certain abilities, and they change your AC, attack, and damage. And your stats don't effect those numbers at all (neither do magic items unless we include the druid robe thing). So you could have a halfling with an 8 str, transforming into a gargantuan TRex with a +18 damage modifier, but still technically has an 8 str.
That being said, many of the polymorph spells do give the character temporary hps. When the character takes damage these hp are reduced until there are none. These temporary HP cannot be "healed" back. So if a character with 100hp normally polymorphs and gains 20 temp HP, then during the fight takes 100hp worth of damage, they are still fine and conscious with 20hp. After the polymorph ends, assuming they took no more damage, they would have 20hp.
So... I was really missing something very obvious, there isn't anything on the book about what happens to your HP because nothing happens, haha. Yes, I was more of a 5th Edition player, didn't play much of Pathfinder 1 or 3.5, but I'm liking this playtest a lot more than 5e, really. Well, thanks for the clarification. Also, this thing about stats is really weird, I mean, Athletics is pretty much the only skill you'll use as a huge animal, but what happens to pure Strenght checks, Fortitude Saves or things that depend on physical atributes in other ways not directly related to Athletics? What if I need the creature's physical stats for other reason, do I need to calculate them based on the attack bonus?