I'm not sure what all the extras in the world were. An interesting case is Anthony Cooper, where he's there but he's a vegetable. Like, he needed to exist there to fulfill John's need to grow up with a father, but his soul couldn't be there because he's so evil, so John gets the memories of growing up with a loving father, but in the present he's just a prop.
Characters like Jack's son and all other people never seen in reality, I would guess he's just a projection. As for Martin Keamy, Mikhail, people who died in the sideways-verse, probably same case as Anthony Cooper, soulless projections of themselves, there because Sayid needed bad guys to kill in self defense.
There's also some weird questions like, what qualifies you to get into the Good Place in this world? Michael kills Anna Lucia to save his son, then sacrifices himself to save everybody who survived the freighter, he wanders the island forever. Ben murders many many people, traps and enslaves many many other people, all for his own selfish desires, then spends presumably a couple centuries second in command in the Hurley dynasty and all is forgiven. And other people absent from the Good Place, Eko and Richard, people who did nothing as bad as Ben. (I'm skipping Walt because his absence was related to the height of the actor).