How to make a 3D camera display in a window

I am trying to recreate the effect that is used in the WithThreeJS extension example, multiple 3D cameras. When you click your mouse the camera changes between multiple different views with bigger or smaller camera screens.

I want to be able to change the size of the camera window without changing the size of the game resolution itself and display multiple

I know how they are able to switch the view of the cameras themselves, I’m just clueless about who they are able to manipulate the size of the camera window without changing the resolution of the game.