How to achieve this "portal" effect for nested scenes

[PavelBoytchev]

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:

How to create a portal effect or how to fake it

portals can get complex, especially entering them. the easiest is most likely drei/portal. a first rough scaffold of the website you linked could be done in a few minutes. if you want this in vanilla it can only be done with a shader. it’s not enough to render a scene into a webglrendertarget using the same camera and project it onto a plane, that would skew it when you turn the camera. the plane has to act as a mask into the virtual scene instead. const PortalMaterialImpl = shaderMate…