Egomoose used a portal effect as suggested by @GoldyyDev - you can tell this when he zooms out 2 seconds in and it apparently looks like hes lost quality in his video but its actually a viewportframe projecting the two images on either side that it sees.
Its not CFrames and its unfortunately very technical and mathematically but fortunately EgoMoose shows how here:
And offers you these two rbxl files which essentially does exactly what your asking. Position it correctly and you’ll have a fully functioning bigger on the inside effect.
Its an easy concept when you understand how its done and if you need help using the above feel free to message me or reply here.
Also you may want to consider memory management when using the above files, Only include in the “World” things that you can physically see in a specific room else you’ll use three times as much memory then you’ll need to in your game as the viewport frames try to render in the entire map.