This is a loaded question because there are several ways to make money on YouTube.
Monetize with Affiliates/Sponsors:  Most of those review channels you see are monetized through affiliates. My channel monetizes solely on affiliate income. I don't even monetize the videos with Adsense. The reason is because my competitors can put ads on my videos. Ads that might be selling the same product with their affiliate OR a competing product that costs me a sale.
Affiliates and sponsorships do blue the line between content and advertising, but it incentivises great creators to make more content.
Monetize with Your Own Products/Services: Many creators monetize their channels by selling their own products and merch. Large companies often sell create content for the sole purpose of driving awareness to their product. They create content that would attract people that would buy what they sell.
Monetize with Google Adsense: This is one of the most commonly known methods for monetizing a YouTube channel. YouTube did recently (2018) tighten the requirements for monetizing your YouTube channel. You now need 4,000 hours of annual views and at least 1,000 subscribers to monetize your channel. You are still allowed to monetize in other ways (affiliates, your own products/services, etc) just not with Adsense.