I'm just playing around with AR core and want to have an object flying around the room and able to land. I'm unsure about how to add colliders to the planes that ARcore is generating based on the visuals on the room. Would this involve instantiating box colliders on the planes somehow?
The ARCore SDK's trackable planes are essentially identified flat surfaces such as the ground or a tabletop.
You can ask the SDK for a list of points for each trackable plane's boundary polygon (retrieved in clockwise order) and create a mesh from those points via triangulation. With the mesh ready, create a GameObject and add a MeshCollider component that references it.
I've created a free Unity plugin that does exactly this. Feel free to use it: https://github.com/jonas-johansson/ARCoreUtils.
I hope that helps!