Since I upgraded to Nougat, I have had this problem of having a third party floating youtube app, but Android would always default to youtube app for youtube links. Normally when you have multiple apps that perform similar actions, Android asks which app should perform those actions. You then choose the app of your liking and can even set it as a default for that task.
Android 7.0 Nougat has multi window features, but it does not quite beat having a floating browser while to do other things. The youtube app I am trying to open youtube links by default is Stream which is on a beta right now. It being in beta might have to do something about Android not prompting an alternative app for opening links. It can also be the developers have not implemented the necessary code to make it seem that way for Android. I did message the developers probably a month ago and got the generic reply. Moreover, since then it has not had any update, so there’s no way to tell if they are doing anything about it.
After ignoring the problem for a while, I was debating on removing Stream as I checked my app usage list and never used it. Then I dug onto settings to find default apps settings on deep inside Android settings.
For this you have to go to: Settings -> Apps -> Configure apps (gear icon) -> Opening Links -> Open supported links -> Youtube
Here you just have to set the “Open supported links” to “Don’t open in this app.”. Now when you open a youtube link from within other apps, it should prompt which app you want to use to open youtube links. Moreover, this is how I am using Stream to view youtube videos. Stream already has an option to open in youtube so you can switch to the official youtube app right away.
Let us know what you think in the comments if you try the method.