Three.js transparent background with a masked scene - three.js

I'm new to coding and everyday I learn something new. I want to have a masked scene with other 3D objects, that are from other scenes, to be behind and around it. At this moment I'm stuck with a masked scene. Right now I have this working. As u can see the background is grey and other 3D objects, that are from other scenes and behind the masked scene, can't be seen because of it.
How can I make the backround from masked scene to be transparent with other 3D objects from other scenes?
I think it has something to do with the fragmentShader, because I can change the background color by changing this line "vec4 background = vec4(1.0, 1.0, 1.0, 0.0);",.
Thanks in advance for all the help!
Update
I understand that the background is transparent on this previous link, but I fail to create a new scene with new 3D objects that is visible and is outside the masked scene. Just helping me make a new scene on that previous link would help me out with my problem.

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