Non-player characters, all characters not run by players, are the rest of the game population.
Most players I ran games for over the years didn’t hire NPCs like link boys who carried a lantern or torch to light the way for the player character adventurers. The player characters would hire NPCS to do things like help them figure out what some item they found might be, a sage to do research in geology or plants; but not take them on adventures.
Some player character groups would hire NPC adventurers to go along with them on the adventure, for a payment of 200 gp or equal treasure share whichever was greater.
If the NPC did a good job, helped out when needed, the player characters would ask them to join the group for equal shares.
Of course, as referee, I ran these characters. I have noticed on one web forum this is not an acceptible idea… I think it works if done well.
I tried to present NPCs for the players to add to their adventuring group. Sometimes the players would turn them down for adding into the group.
As the player characters got up to levels they attracted henchmen, we would work together to decide whom arrived to support them. One player character group obtanied a small wood manor house, which they turned into a stone castle, added walls, towers, and a gate house.
Most of their henchmen worked there and didn’t go on adventures.