I am a new iPhone user and just bought an iPhone 6 plus. I have been using Android (and continue to do so) where you can choose default apps to launch when you do a certain task, like launching YouTube from facebook when you click on a YouTube video, or launching a web page directly into chrome instead of the poxy facebook browser. What is the point of all these apps on the iPhone or iPad if they can't speak you each other? It's very frustrating especially if you want to cast YouTube to your TV like I do sometimes.
I have a couple of thoughts on this.
First, FaceBook and Google (YouTube) are different companies, each with their own preferences and practices - which means that there may be no rhyme or reason for why/how things are done within and between their apps. This is true, even for combinations of apps/vendors that may work or interconnect better on other platforms (Android, Windows Phone,
BlackBerry).
Secondly, iOS requires apps to be "sandboxed" for security. This means that iOS blocks some of the interaction between apps to protect you and your data. It’s part of Apple's way, and it’s baked into the system. This is a case of "Apple's way or the highway". They have a mechanism for allowing data between apps in some cases, which requires you to give permission for the interaction. In other cases, it’s just not possible.
Finally, Apple does not allow third party software to become "system default" for services that Apple itself provides. For example, you cannot use a 3rd party web browser as the default because Apple sets Safari as the default. Same with mail clients and Mail. Again, it’s just how it is - Apple's way. It’s part of the "walled garden" experience, designed to protect you and your data.