Hey, that “ViewportFrame guy” here! I gave this a go, once upon a time.
This is actually almost impossible to do in Roblox. You’d have to rely on buggy behavior that we’ve been explicitly told not to do.
I haven’t watched the video, but I think I know what it’s going to say. At some step, At 4:10 in the video, it’ll discuss how you have to “crop” to a specific portion and make it fill up the frame. We don’t have the ability to do that. However, you could achieve that using Glass rendering bug and some resizing math. It won’t look great, but it works in theory. I don’t recommend it. Because it’ll look off, players will notice when they get close. They get close at crucial points in gameplay, so this isn’t yet viable on Roblox.
Edit: Perhaps one day ViewportFrames will be able to render other ViewportFrames inside themselves. If they do, you could use techniques similar to my image clipping module to crop and size the relevant image. Until then…