I’m going to apply the plug-in I made to the game project and package it.
Which folder should I add the plug-in to?
Is it the Plugins folder of the game project? Or is it the Plugins folder of the engine?
Take a look at this resource: Plugins in Unreal Engine | Unreal Engine 5.0 Documentation
Plugin Folders
In order for Plugins to be found, they must be located in one of the search paths for Plugins, either in your project, or in the Engine itself.
Plugin Type Search Path Engine /[UE4 Root]/Engine/Plugins/[Plugin Name]/ Game /[Project Root]/Plugins/[Plugin Name]/