Question Why do both exist that OnApplicationFocus and OnApplicationPause?

it looks same functions. the difference is only
when game started first:
call OnApplicationFocus(true) // after Awake(), before Start()

but if it need something when game started first, just do coding at the end of Awake().

 

There are two different event methods because there are two different interactions. What these interactions are depends on which platform and whether you're running in a build or within the editor. OnApplicationPause called after an Awake is just one of the things that it does.

https://docs.unity3d.com/ScriptReference/MonoBehaviour.OnApplicationFocus.html
https://docs.unity3d.com/ScriptReference/MonoBehaviour.OnApplicationPause.html