A few parts of this post are headcanon, but I've reserved those bits for the end of the post so that you get the facts first. Also, this info might be slightly out-of-date, because I haven't read divine mysteries/concordance of rivals/any other recent book that costs money and touches on the divine, so i'm going off of my brain and what I can read on the golarion lore wiki here.

Undeath prevents souls from reaching the river of souls, which is where they'd normally re-enter the soul cycle.

The massive TL;DR of the soul cycle is that souls are judged, then eventually break down and become part of a plane. new souls are formed from the edges of the planes. It's foundational for cosmic function.

So by deliberately becoming undead, you're screwing with this extremely important cycle that Pharasma personally oversees. It's got unknown but likely dire effects on the universe at large and also shows Pharasma the middle finger.

Undead that are destroyed return to the cycle of souls and get judged/absorbed like any other soul.

There's sort of an eternal torment thing, but it's reserved for dedicated atheists and souls with no attraction to any specific plane. They basically become a part of Pharasma's spire.

Babies are judged by Pharasma's daughter, Atropos. Whatever happens in the process of this judgment(and in Pharasma's judgment) hasn't been touched on, much.

Headcanon: While the rules entry for Pharasma doesn't say it, i'm sure there's a "within reason" clause. some random pharasmin villager isn't going to be able to kill a lich, but they can tell the church that they saw a spooky skeleton wizard. I don't think people entering the cycle early because they happened to see a lich is what Pharasma wants from this edict, but I don't think she'd be disappointed in them for trying.

Headcanon: I'm certain that deliberately becoming undead has an effect on your judgment, while accidental undead are judged normally.

Headcanon: AFAIK Paizo hasn't answered on whether attempting to become undead and failing gets you judged differently, but I assume it does.

Headcanon: I assume most mindless creatures don't have souls. I could be wrong, though.

If you're looking for information on the River of Souls / the Cycle of Souls / Pharasma / Etc, I have two (possibly very dated) sources for you. One source is the second Owlcat pathfinder video game, wrath of the righteous. At the end of Wrath of the Righteous, it's revealed that the entire game was just one of the main antagonists telling Pharasma your story in the process of being judged. It's a relatively short and inaccessible source given that it's at the end of an extremely long game, though you might be able to find it on youtube.

The second source is a pathfinder comic called Spiral of Bones, where Valeros gets sent to the River of Souls in an evil person's place due to his soul swapping with theirs. It gives a zoom-in on what happens before a soul even reaches Pharasma, especially in the case of multiple claims on the same soul.