To add enemies in a game, place their prefabs to the Game Manager and then set the parameters:
Enemies' movement behavior in a scene
Random: one random point from the Spawn Zones will be chosen.
Specific point: set a spawn zone for an enemy as you need.
Spawn enemies constantly.
If ∞ is not active, this number means how many enemies will be spawned during the game.
If ∞ is active, this number means a limit of enemies in a scene. (if a player kill 3 zombies, 3 more will appear in the game)
A break between the appearance of enemies (in seconds).