https://support.google.com/youtube/answer/6175006
The reality is it takes so much time and energy to produce content and the bar is high that creators make it their full time jobs.
So either, they have YouTube monetization with ads or they do in-video ad rolls, "sponsored by Squarespace", yadda yadds. Or they literally include some product into their video and it acts as a hidden ad because they generate a story around that product.