Virtual File System API?

Most game engines usually implement a virtual or abstracted file system and provide an API for that, but I can’t seem to find any real documentation on this in Unreal.

I did find a Wiki entry by Rama talking about some very specific stuff, which used FPlatformManager, but a search of said name returns nothing in the API documentation.

This is a pretty key part of an engine so surely it exists, so where is the documentation on it?

Well there’s the IPlatformFile interface, and IPhysicalPlatformFile, which is the base class for the platform specific file system implementations. They can be found in Core/Public/GenericPlatform iirc.
IFileSystem::CreateFileSystem() will hand you an instance of the correct implementation for your platform I think. The API is very straightforward, easily figured out by looking into the header files.