As far as I know, "YouTuber" is the correct way to write it, but you'll often see "Youtuber" and "youtuber" on the web because someone made a typo or they're not familiar with English capitalization rules. As far as I know, they all have the same meaning.
Actually, I think the most common way to write it is this: "Fall" from "The Four Seasons" by Vivaldi.
After that: "Fall" from Vivaldi's "The Four Seasons"
After that: Vivaldi's "L'autunno"
As for "Fall" of "The Four Seasons" by Vivaldi, that's not really correct. You should use "from," not "of," to connect the smaller work to the greater work. Otherwise, it sounds like "Fall of the Four Seasons" by Vivaldi.