As you saw, the 'Dropbox-API-Select-Admin' header supports two different "modes": "Whole Team" and "Team Admin". These aren't something you can choose between on the client side though. Each API endpoint is written to support one of the modes, and you can't change which mode that is from your app.
You can check which mode is supported by any particular endpoint in the documentation for that endpoint, under the "AUTHENTICATION" section. For example, the /2/files/get_metadata endpoint lists support for "Dropbox-API-Select-Admin (Whole Team)", whereas the /2/files/upload endpoint list supports for "Dropbox-API-Select-Admin (Team Admin)".
I also recommend reviewing the Namespace Guide for more information on how to access various parts of a Business team.
