It may take some time to learn what the differences are. The layout becomes familiar quickly but I confess I didn't find it terribly intuitive right off the bat.
There are definitely things in the app which are not available on YouTube; First Watch content, some content which cannot be posted on YouTube, free access to the monthly webinars, and, if memory serves, some of the training which otherwise needs to be paid for.
And, of course, getting content bound for YouTube a few days early, with responses and conversation from a more thoughtful, focused crowd. Makes it easier to sigh and scroll on when the... ahem... less eruidite followership shows up on YouTube :-)
Thus far the only thing I've discovered not available on the mobile app are these forums -- I suppose there could be more, but I think everything else is same on phone and computer.