The NPC Populator for Unity makes it easy and quick to add many free-roaming Non-Player Characters to your environment without writing any code. Only some configuration is necessary.
Included is an NPC Factory to automatically generate random humanoid NPCs based on humanoid models that you provide -- many from the Asset Store will work -- and scripts to animate your NPCs while they navigate through your environment. In version 2.0, the NPC Factory has been completely redesigned to be even easier to use than before.
Navigation uses Unity's Navigation Mesh system along with a waypoint system that is provided by the NPC Populator.
Other improvements in version 2.0 include validation that the package works in Unity versions up through 2021, NPCs can pause at waypoints, the package can be customized to use third-party animation clips, a new Waypoint Assistant tool is included to semi-automatically place waypoints in some scenes, and a few NPC character models are included.