How to read dnSpy code from assembly_csharp.dll

So I have managed to retrieve the assembly_csharp.dll using the il2cpp tool from an android apk, however my issue is that when I go to read the dll using dnSpy I have all the correct looking function names, but the body of all the functions reads like:

return null; 
return default(bool); 
return 0;

etc etc

My question is is there any way to retrieve these functions, or otherwise how can I modify this code to change the app’s behaviour