How do I separate calls and media volume on iPhone?

Your reply is wrong and has nothing to do with what OP said. Can you please read again and then answer ?

and to OP, sadly you can’t do that. iPhone is too limited in features like that. Somehow iPhone is still stuck in 2007 while others are in 3024 with their tech and user friendliness…

Unfortunately, there isn't a way to directly play a video for your viewing through another app while on a FaceTime call.

If you are referring to SharePlay, it requires everyone on the call to see the same thing.

Here's how it works:

• Everyone on the call needs to be using an Apple device with iOS 15.1 or iPadOS 15.1 or later (SharePlay isn't compatible with the web version of FaceTime).
• Start a FaceTime call and then tap the SharePlay button.
• Choose "Share My Screen" and then navigate to the app that has the video you want to watch together (like Apple TV, Netflix, etc.).
• Once you start playing the video, you'll be prompted to either "SharePlay" or "Play for Everyone" - choose the option that allows everyone to see the video.
• The other person on the call will see a prompt to join SharePlay, and once they accept, the video will be synchronized for everyone so you can watch it together.

Here are some additional things to keep in mind:

• SharePlay only works with apps that support it.
• You can control playback (play, pause, fast forward, etc.) from either device during the call.