Fix missing UnityEngine codes for DLL/mono backend games

Came across a game that you can't add the mod menu due to missing codes from UnityEngine? Because it was stripped out but it is fixable

This happens if UnityEngine has been stripped out

To fix, you need to download original unstripped Managed libs I obtained from Unity Editor.

First, get unity version. Follow the tutorial:

https://www.andnixsh.com/2019/05/how-to-get-unity-version-from-any-asset.html

Download Managed dll folder. Explained in this thread:

https://www.andnixsh.com/2020/04/unity-mono-stuff-for-android.html

Replace all managed dll files to the Extracted folder, and reload all assemblies in the dnSpy

It will not be red anymore. Now you can compile

Replace all managed dll files to the game's Managed folder as well