Have you looked in the Settings/Default apps/
and see if youtube is mentioned there?
if it is, and it is using a browser instead of the app you want, delete the default.
Then the next link you get, it should ask how you want to handle it, and hopefully it offers up the YouTube app. If that happens, make sure you click on that Once only, then click Make Default.
I just looked at the YouTube app itself and no where in Settings could I see a way to make it default when an external link is used.
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2 ea. Verizon Galaxy S5, 4.4.4 forever!, ART
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