If you are going to use logic bricks only, maybe you should consider replace mesh!
If you have troubles or can’t use coding(which would simplify things), plan well how things can work with logic bricks.
In the case the player has to pick a part for a, lets say vehicle, and then attach it to the vehicle, you can prepare the vehicle beforhand:
The vehicle has all the placeholders (empties)for the parts in place, every placeholder will have a base mesh with zero no rendering(can be zero vertices or an invisible cube). The place holder can then have an identifying property and the constraints. If you cant make add the object and then the constraints, then you can set them manually and hide the vehicle instead of spawning it from a hidden layer.
The object or pick up can then have 3 state, iddle, active, used.
Iddle, the object is laying around with no logic or physics enabled.
As it’s picked up, it becomes active. You may parent it to the player’s hand for example. Then you have a near sensor running looking for this placeholder it belongs to, if found it sends a specific message, announcing itself.
Ased: as the player places the object in place, various things happen: while the object is sending messages, the vehicle is activated as the player is next to it or has previously activate it or it’s the main vehicle, you decide. It receives the message, replaces the default mesh with the announced one and sends a terminate message. Upon reception, the object/part or pick up is ended, deleted.
Remember, simplify all your moving/working parts of your objects, use empties when possible, then “mask” it with a simplified mesh. Don’t try to parent your parts directly to the object and then try to make it work as a compound, it will increase you physics calculations.
Get yourself a screen recorder, it will considerably increase your screen capture quality. If your work is of quality, the other artists will take you seriously, for it shows your project is strong and well planed/structured. Don’t animate unless is necessary (that means, you don’t need to show your weak points, if you wont use a good model, have a simple cube with linear velocity moving around) the glitchy animated doll is amusingly unappealing! When shooting get a good angle that shows as much of the action as possible! Show some hard work…
Plan ahead a bit more. Planning is like a map, the more detailed it is the better! Many times I tried to go with the ideas I have in my mind, but as the days passed, the ides change, you can easily change you mind and your project changes directions till it’s in a cul-de-sac! Lay down on paper all your ideas, if they work there, there are chances it will work on screen.
Try to stick by your plan, don’t look for shortcuts, ask for help when stuck. Asking help works better if you have worked on finding solutions, make sure to state what you want to accomplish and what you have tried. Then the community will help. If you decide that the help given is not satisfactory, then maybe your message was not clear, try to give more details. If you take heed of peoples advice, they are more prone to help you next time!
Good luck! And stay strong!
Edit: One more thing: you can edit your posts by clicking on edit post. People read your post if they are interested, be it on top or not!