It uses all the front sensors. Both the ones that look straight ahead and the ones on the side of the bumper that are used for parking. It warns you if any sensor gets close to another object.
No, it does not use the ones on the side. Those are a totally different type of sensor.used by the active park, they are not used by the park aid (beeping). But the park aid sensors do have a field of view beyond what is just directly in front. Just like the rear sensors will pick up the sides of my garage door, the front ones can pick up objects slightly to the side of the car.
As an aside, note that Ford has made vehicles with just front park aid (09-12 Lincoln MKS) that did not have active park or it's sensors and also makes vehicles with active park (Focus) that do not have the front park aid sensors.