How can i make a drag and drop builder

I was making a building game where the player drags parts from a list and moves it to the car and it will attach to a certain point. If you dont get what im saying just check out this video -(How To Build an Interplanetary Rocket In Kerbal Space Program Using Only 7 Parts - YouTube) - i want those same building mechanics but i have no clue on how to even start with those. I already have my parts lets just say for simplicitys sake that the parts are called part1, part1, and part3. How would i get it so it displayes those parts, allows the player to click on it so it makes the part and allows the player to move it over then allows the player to attach that part to a certin place on the car. ((just so you all know in that video was a game called kerbal space program and it was made in unity it uses empty game objects so make those attach nodes))

When you click a GUI button it Instantiates your gameObject and it follows your mouse, when close to a snap point it snaps to the snap point, which is a Vector3 point.
When you click again it attaches to the parent gameObject.
Check the links;