CSS3 transitions have some significant limitations that make them unworkable for a serious animation platform. They don’t provide precise controls over the timing or easing. They’re great for simple effects but the GreenSock Animation Platform delivers extremely precise rendering, so you can do things like pause() and reverse() an animation anytime or skip to a specific time and play from there, etc. Try creating a CSS3 transition that uses an elastic.out or slow motion ease and then jump to 0.72494-seconds into a 2-second transition and pause() only to resume() later. It’s impossible from what I understand. So no, the platform doesn’t make use of CSS3 transitions. However, it is highly optimized for performance. See the detailed cage match where GSAP battles CSS3 transitions where there’s a detailed comparison in several categories.

Yes.

Yes, Draggable is just for DOM elements. But the real magic behind all the fluid motion and snapping is InertiaPlugin, and that can be used to tween any property of any object, not just DOM elements. So yes, you can absolutely get this kind of motion in other contexts but you'd need to wire up the actual dragging logic yourself and then fire off an InertiaPlugin tween when the user releases their mouse/touch. InertiaPlugin can even track the velocity of any property for you too (even function-based getters/setters!), so it's quite a powerful tool.