ummmm not sure about the how is attaches bit but I guess its probably like a leach LOL As it develops it probably just latches on to where ever it lands first?

my understanding of the how it moves bit is not so much that it moves but as the uterus stretches with your growing baby, the placenta moves as that spot it is attached to moves KWIM? like picture a balloon thats deflated with a dot on it. As you blow the balloon up the dot will appear to move as the rubber stretches. At least thats how I thought it worked....