How do you create global variables in a C++ plugin?

Multiple ways I suppose. I am stating generically as I don't know your specific context/usecase

Staying within the InDesign SDK constructs you can use the following

• Add non persistent interface to boss classes like session, workspace. Set and get them as per your needs

• Using IBlackBoxCommands which can store key/value pairs

Outside InDesign SDK

• Something like a singleton class which is set/get as needed and accessible in any code file

-Manan