Income from YouTube is usually earned in one of, or a combination of, four different ways.

Adsense

YouTube will run ads on your videos and share the advertising revenue with you. Under YouTube’s current rules, you will need 1,000 subscribers and 4,000 hours of watch time on your channel. You will also have to go through a review process before final approval.
According to Social Blade, there are many YouTubers earning well in excess of $1 million annually from Adsense. These folks have hundreds of thousands of subscribers and are getting millions of views on their videos.
In order to earn significant money through Adsense you will need a huge number of views because your earnings are calculated on a “per thousand Views” basis.
For example, if you earn $3 CPM (per thousand views) and you have a million views, you will $3,000 (1000,000/1,000 x $3 = $3,000).

Affiliate marketing

As an affiliate you promote someone else’s product and earn a commission for each sale or action. The beauty of affiliate marketing is that you don’t have to create a product, ship a product or deal with any customer support issues.
You just need to find products that are relevant to your audience and will help solve a problem encountered by them.
You can find affiliate programs for any niche you can imagine. Do a google search (“your niche” + affiliate program) and you’ll have no trouble locating many programs.
To become an affiliate marketer on YouTube doesn’t require any approvals. You can market products as an affiliate on your very first video if you choose to do so.
Depending on your niche and the products you market, it’s very possible to earn much more as an affiliate than from Adsense. Of course, you can do both!

Sponsorships

If you develop a large enough channel with authority, you may be offered a sponsorship opportunity.
In this scenario you may create an entire video, or a portion of the video, promoting someone else’s product. Normally you would receive a flat fee as your compensation.+
Depending on your channel’s authority and the effectiveness of your promotions, you can do this over and over for different advertisers. The fees can be lucrative. I know of one YouTuber who received $50,000 for one sponsorship deal.

Merchandise

If you become recognized, you can create your own branded merchandise and sell it to your viewers.
There are YouTubers who have created entire branded clothing lines and market their merchandise in all their videos. They ve created online retail stores and refer their viewers directly to the stores.
If you publish your own courses, YouTube is a great vehicle to use for promotions. You already have an existing audience who follows your advice regularly. It only makes sense to market to them.