I had always been under the impression that Golarion and the negative energy plane were on entirely different metaphysical planes. This would mean that you couldn’t simply move from one to the other using mundane means.
The Tar-Baphon entry on Lost Omens: Legends says (pg.104):
Tar-Baphon dug a portal to the Negative Energy Plane on the Isle of Terror …
To me, this sounds like he physically dug a hole to the negative energy plane. At face value, this would only be possible if the negative energy plane were somehow underneath the earth of Golarion.
I have access to many of the PF2 books, but am weak in lore published in PF1 resources. Is it true that the negative energy plane is literally below Golarion?