It is indicated here that "Silver Line is FREE from Logan Airport inbound to South Station (including a free transfer to the Red Line)."
First, how does the free transfer work? Do I need to have a Charlie card and tap it upon boarding a Silver Line bus and then upon entering the train (or bus?) on the red line? If I don't need to use the red line, do I need to take any actions (e.g. tapping a Charlie card) when boarding a Silver Line bus?
Second, is it free to travel back from the South Station to Logan Airport?