The basic premise of e-commerce is that you are going to have a product that you can obtain inexpensively from some source, which you then sell online with some markup on the price. Then you pocket the difference. Out of that you'll also pay for advertising, website costs and other costs of your business.
This can also be done with digital products - ebooks, videos, online training courses, music and more. Indeed those are all forms of e-commerce.
But when we talk about e-commerce or Ecom, the main focus is mainly physical products that people use in their everyday lives. Gizmos, gadgets, clothing, electronics, auto parts, home appliances - that's just a partial list of the kinds of things that are sold online and qualify as e-commerce.
And the main strategy that makes a lot of people rich in e-commerce, is drop-shipping. That's when you never touch the product, you simply accept the customer's money and then order the product from a cheaper supplier, who then ships it to the original buyer. The price will be much cheaper so you'll have some profit to pocket.