I know that there is a lot in the docs but I recommend you to go trough all the sections to get familiar what are the possibilites of the framework. Later you can get back to a specific section and learn how exactly the implementation looks like in detail.
Another good idea is to use the search functionality of the forum. You will get answers on nearly all your issues you coud face while you are slowly mastering the framework.
So here you go for now:
https://doc.babylonjs.com/features/featuresDeepDive/scene/multiCanvas
https://doc.babylonjs.com/features/featuresDeepDive/scene/multiScenes
https://doc.babylonjs.com/features/featuresDeepDive/postProcesses/renderTargetTextureMultiPass