IL2CPP

"The IL2CPP backend converts MSIL (Microsoft Intermediate Language) code (for example, C# code in scripts) into C++ code, then uses the C++ code to create a native binary file (for example, .exe, .apk, or .xap) for your chosen platform." - Unity Manual

MSIL contains various type information to allow for C# introspection.

Various tools can be utilized to recover this type information.

IL2CPP Dumper

BepInEx

https://github.com/BepInEx/BepInEx
Bleeding Edge Builds:
https://builds.bepinex.dev/projects/bepinex_be

Backup the game's root directory
Extract all of the files directly into the root.
Edit './BepInEx/config/BepInEx.cfg' to set 'LoadDumpedAssemblies = true'
Run game binary.
If everything works correctly './BepInEx/unhollowed' should contain the unhollowed dlls

Open the unhollowed files in dnSpy. These files should contain both type information and code.