RE: Where does FAA airspace start?
The more correct answer is that there is no such thing as "FAA airspace". There is the airspace of the United States of America and the FAA has authority over all of that. Some of that airspace is under positive control and can extend to the ground in many places. But controlled or not, the FAA has authority, granted by Congress, over all the airspace of the USA. In other words, the FAA can make rules regarding who does what and how in the airspace of the USA.