You can create instances of a MonoBehaviour using instantiate or via AddComponenet on the GameObject
Instantiating Prefabs:
https://docs.unity3d.com/Manual/InstantiatingPrefabs.html
AddComponenet:
https://docs.unity3d.com/ScriptReference/GameObject.AddComponent.html
Aside from that you could create a non MonoBehaviour class that has a method suitable to be called as a coroutine but you will need to call it from a MonoBehaviour e.g. in your update call
This assumes myClass is an instance of your class and Foo is your suitable method.
Beyond that you can use BackgroundWorkers and manage your own threads if you want truly multi threaded code however be aware that many aspects of the Unity engine are not thread safe ... if you want to get into multi-threading maybe checkout the Job system first as its meant to make such simpler ... not sure I agree its simpler but it is safer :)
Instantiating Prefabs:
https://docs.unity3d.com/Manual/InstantiatingPrefabs.html
AddComponenet:
https://docs.unity3d.com/ScriptReference/GameObject.AddComponent.html
Aside from that you could create a non MonoBehaviour class that has a method suitable to be called as a coroutine but you will need to call it from a MonoBehaviour e.g. in your update call
StartCoroutine(myClass.Foo());This assumes myClass is an instance of your class and Foo is your suitable method.
Beyond that you can use BackgroundWorkers and manage your own threads if you want truly multi threaded code however be aware that many aspects of the Unity engine are not thread safe ... if you want to get into multi-threading maybe checkout the Job system first as its meant to make such simpler ... not sure I agree its simpler but it is safer :)