Who told you that ?? Your question is wrong or otherwise your answer is - You are using youtube for watching videos and at the same time you get notification from gmail or wattsapp .. if multiple app cant use internet at same time then how you are able to get notification from wattsapp or gmail ? Because without internet you cant get any notification from these app. I hope you understand
All the apps take data when internet connected on the phone. It's not like only one app will use the internet. Let me know more clear with your question and I can help you more
