The money really comes from sources outside of youtube. Things such as sponsors, partnering, or affiliations. One simple example would be an Amazon associate relationship. Links to your products that you place in the description, which can be purchased on Amazon. In addition, if the buyer purchases anything else on Amazon if they come through your portal link, also earns you money.
My original youtube channel that I shut down about 3 years ago had maybe 150+ videos with about 60K+ subscribers and a few million views. I think it would monetize around $250 per month, but again, as an example, the real money would be linking all the gear, accessories, etc, seen in the videos to an associate link like Amazon. Then, of course, you can review items in video's and also link them. If you get big enough you can start looking at sponsorships, etc. In turn, all of this could lead to other business opportunities. As an example, I would get a lot of inquiry on my business side of things about events, product R&D, T&E, consulting, not to mention a lot of people wanting to send me a lot of good stuff but mostly crap to review and want a video out of it. But I avoided this.
All in all, get a good base and look outside of simple monetization.