uploading files to a Team folder

Regardless of what folder you're uploading to, you would use the same upload methods, such as UploadAsync in FilesUserRoutes on a DropboxClient. There aren't separate team client methods for uploading to team folders. You can upload to whatever folders (team folder or not) that the connected user account has access to.

There is some additional configuration needed if you are trying to access team folders in a team space though. Please refer to the Team Files Guide for information on that.