When the dinosaur protrudes from the portal in the Encounter Dinosaurs app, it appears to be lit by the real room lighting, just like any other RealityKit content is by default.
When the dinosaur is inside of the portal, it appears to be lit by the virtual environment, and the two light sources seem to be smoothly blended between at the plane of the portal.
How is this done? ImageBasedLightReceiverComponent allows the IBL to be changed on a per-entity basis, but the actual lightning calculation shader code seems to be a black box, and I have not seen a way to specify which IBL texture is used on a per-fragment basis.