I want to make sure when I cast YoutubeTV from my iPhone to a friend’s Roku at his house that it doesn’t reactivate his YoutubeTv account (Which he stopped) and charge him. If it is casting from my device to his Roku, it’s my account, right? It confuses me that it goes to the Roku YoutubeTV app. Am I understanding what’s going on here?
Re: Casting YourubeTV to friend’s Roku from my IPhone
TL;DR - Don't cast from within the YT TV app, screen mirror from Control Center.
Your confusion is warranted considering the (loose) usage of terminology.
"Mirroring" is when one device's screen is directly reflected (with a delay) on another device.
"Casting" is when an app on one device opens/controls the same app on another device.
The problem arises when the two terms are used interchangeably, or both types of functionality are mixed into one protocol (e.g. GoogleCast protocol, or AirPlay protocol), or the name implies the opposite (Miracast is mirroring, not casting)
Roku supports DIAL (casting), Miracast (mirroring) and AirPlay2 (casting and mirroring).
YouTube/YouTube TV uses Googlecast (casting and mirroring) and DIAL (casting), but since Roku only supports DIAL, you can only cast (DIAL) to the Roku's YT TV app (meaning you can only remotely control the Roku's YT TV app using the iPhone's YT TV app).
However, If your friend's Roku is 4K/HDR-capable, it should support the new AirPlay2 functionality, and you can use your phones AirPlay2 screen mirroring functionality to mirror the screen, instead of YT TV's internal cast functionality.