It's NOT easy, but it is very profitable if you know what you are doing. And yes, there is a good potential for passive income. I find most Internet Marketing coaches are bogus, and just want your money. I learnt from someone of very high integrity called Eric Holmlund and he offers is teaching 100% free. His main angle is marketing digital products. I wrote my impression of him and his tips here if you're interested
The advantage for Google was that the user-uploaded content really only cost the company in terms of infrastructure maintenance. The idea seemed to be that the cost of maintaining YouTube would drop as technology improved and more video content could be monetized through ads and market pricing. Thanks to some innovation within the ads themselves, the market for the ads has improved. YouTube overlays allowed ads to populate videos at different points of the viewing rather than just having a pre-roll ad, and they followed the embedded video around the web. Even with better ads, however, as of May 2015, the revenue was still not enough to pay for the site.
How much will you make? I think my best month with Google AdSense was almost $5,000 over the last ten years. That amazing month blew my mind since it was actually near the beginning of my blogging journey. When you go from making zero to $5,000 in a month, that will rock your world. For me, it also got me even more excited because I knew there were other ways to monetize.