Your business is a client. Content mills are agencies that build up a pool of clients and a pool of writers. You develop topic ideas and submit them to a public pool, a specific team pool, a "love list" of vetted writers, or to individual writers. A writer in the group claims the topic, writes it and submits it for your review. You order a piece of content, pay for it (paying a bit extra for the content mill to skim off), and can use it on your site. Content mills are extremely hit-or-miss in my experience with them over the years, with more misses than hits. The article writing quality will often vary wildly; one writer can be excellent, and another is unusably bad. Content mills generally aren't worth the time and money since you have to handle all of the extra stuff yourself. Some businesses can use them, especially if you have a tight budget and want a lot of the details managed for you, but the niche is narrower than many people think.