How do you make the material inside the window visible from the outside?

It’s the material used for “RoboRecall”.
Who knows how to make this?

Or, I want you to let me know the hint to make the material inside the window.

I am not sure if I understand your question right but to look inside the wall you just need a mask for the windows or an alpha channel

Personally, I never tried it, but you want to google for “cubemaps in ue4” or something. This answer might be the solution already. Would be great to hear if it worked.

Your answer was very helpful.
Thank you~^^