Can someone clarify the Paypal Subsciptions options for me?

I am trying to create a free and paid membership. When users sign up they default to a free infinite subscription. I set up Paypal express acct. and gateway and applied to paid membership. But even though both subscriptions are activated I don’t see upgrade option on member account page with shortcodes.

I also dont get what the difference between the Express and Single payments gateways are!