Hi,
no worries, these things takes time to properly frame in your mind.
you tell it to refer to a given point in many ways.
- you can parent them all to that point, which is the easiest, because then you simply need to query their local position and they will be express relative to their direct parent
- or you have them freely in the world, and at the time of querying for their position, you use InversTransformPoint, to get the local position of that object compared to the reference object.
for you, I think parenting might make a lot of sense and be easier to deal with across the board. I do that myself for AR, I have a placeholder prefab with a shadow plane, this placeholder moves on the AR plane, then when I want to show something I instantiate it and parent it to this placeholder, then I can don't care about AR anymore, I just work inside the placeholder, and I always get a good shadow underneath, etc etc.
Bye,
Jean