Simple, Pathfinder devoted an entire book to them.
Back in 2e D&D, there were books such as The Complete Book of Elves or The Complete Book of Dwarves that fleshed out the player character races for the players. Ditto for 3e D&D's Races of the Wild or Races of Stone. This practice was dropped by the time 4e came around and has not been resurrected by WoTC as far as I know.
Pathfinder 1st edition kept this tradition going by coming out with it's own racially specific Companion books such as The Kobolds of Golarion or the Blood of Angels, which I summarized in another forum thread on Enworld.org.
The Aasimar in 5e need a more diversified origin as there are quite a number of Celestials besides the angels.