You cannot directly control another person’s screen on FaceTime
You can ask them to share their screen during the call
You can guide them verbally while they control their own device
You can use FaceTime SharePlay for shared viewing of supported apps and media
You can use Remote Management or screen-sharing apps outside FaceTime if both parties agree and the app supports it
You can use Apple’s accessibility features like AssistiveTouch or Switch Control on the other person’s device only if they grant access and set it up themselves