Thanks for reaching out, welcome to the community.
You can connect your QuietComfort 35 II headphones to your iPhone via Bluetooth or via the provided 3.5mm cable if you have a headphone jack available. By doing this, it will play the audio from your mobile to your headphones, this will include audio through apps such as Youtube and Netflix. I would recommend if you are watching video with your headphones to use a wired connection as this will avoid any audio to video delay which can occur with Bluetooth connections.
If you have any further questions, do let us know.