It should be in the left-hand column of page links:
If you don't see it perhaps you're a moderator without permission for Billing. Or, if you don't see the Admin set of pages at all maybe you're logged in as a member, or not logged in. Or, if it says "Billing" then you're looking at a Premium group.
Correct. Premium groups have 20 GB storage instead of 1 GB, and they have various other features like Direct Add that are not available to legacy Basic groups. In my screenshot note the list of "Legacy Enabled" versus "Not Enabled".
That too is correct.