LiteBrite, one of the popular ones, indicated about a year ago that they made around $7k/mo just from YouTube monetization. At the time they were technically homeless and just had a tow rig, trailer, and toys. Since then they've bought a bunch more stuff, including an acreage in Utah and brought two more people onto their channel. No doubt they're also getting sponsorships and free parts.
So yeah, suffice it to say they're not funding it from a day job like a hobby.