Selling products to people who have subscribed to an email list may be a great way to get customers. But part of the secret sauce is getting people to be on you

Selling products to people who have subscribed to an email list may be a great way to get customers. But part of the secret sauce is getting people to be on your list in the first place. I was hoping the article would touch on that a bit more. Where do you find people who are potential customers who willingly want you to market to them? If I could master that, the sky is the limit.