I dont know how larger games do it but it mostly depends on what kinds of enemies and what genre of game youre doing.
If you have a lot of enemies that will be spawned and despawned and they are mostly the same you can do an object pool where instead of destroying the object it gets hidden and added to the back of the pool for another enemy to spawn in as in the future by showing it and moving it to the correct spot
In terms of when to spawn it usually (assuming youre doing most genres) you can just spawn it right outside the view of the player when they hit a trigger. In games I usually make enemies are spawned on a timer since it tends to be more arcade like and in that case you usually just spawn them outside the range of the player in a random location around a radius after X amount of time has passed