Back and front are powered by the same battery which typically lives in the seat post. When the battery gets too low, the front shifts are disabled first as they take more juice.
Not on mine, SRAM etap, there is a small rechargeable battery in the front and back. They are the same so can be swapped.
I really, really like it. I know I paid way too much for that bike but it is amazing and I can really feel the difference in a way I never could before.
