How to achieve this "portal" effect for nested scenes

I’m trying to recreate this portal effect i came across on this page :
Any idea on how this is created ?

I have no idea how this specifically is created, but if you look for portals in the forum, you will find quite a lot of posts about them. You could start with this one and its demos:

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that’s a classic enter-portal, among the sandboxes that @PavelBoytchev linked this one will do

the enter animation differs slightly in that the portal itself grows to capture the full screen whereas in the box the camera goes in. if you want it exactly like that all you would have to do is increase the mesh scale that holds the portal material.