SEO, Google Ads, and Facebook Ads change every six months. Outdated courses are worthless. Free courses are great because they are often "living documents" updated by instructors to stay competitive.
Before we dive into the "how," it is vital to understand the "why." Udemy operates on a marketplace model. Individual instructors create the content and set their own prices. There are four primary reasons instructors offer courses for free: free udemy courses