I actually have more than one favourite YouTube channel for several different niches.
For writing I like Jenna Moreci because she’s entertaining. I also like the Story Grid channel and Bookfox because they give great writing advice.
For fitness I like the Buff Dudes channel, quite entertaining
The Alston Godbolt channel, particularly his I Tried It series, where he actually tests the side hustles that other make money YouTubers do to see if it’s actually worth doing and if not gives a better option
I also like Kamikaze Cash and his Freeloader Challenge series where he goes through various different free options for free money sources and ranks them