Each course is taught by an expert instructor who has created the course. Udemy hosts the courses created by thousands of instructors from the world over. The website does not charge anything for hosting the course. This means that you can host as many courses as you want on Udemy without having to pay a single cent. The price of your course can range from $10 to $50.
You may not know but you can rent your website if you have good traffic. I learned this method of making online income when I was working with a client in the real estate domain. Rather than building out his website from scratch, he rented out a website from a guy who already had the traffic, with people interested in buying a home in his local area.
You can pick writing jobs directly from a live job board or have to bid on a job and hope that the job is awarded to you by the relevant client, depending on how the site works. Some sites may require you to take an English grammar and writing test and some may require you to submit a sample or samples of your work while others may require you to take a grammar test to evaluate your skills. If you pass any relevant test, the relevant site will let you take on writing jobs.