Freelancing is just another way of saying running your own business. You market your service through different sources, agree on contract terms and perform your work for clients.
Udemy isn't necessarily a freelancer site but a great place to launch your side hustle in video courses. Freelance for more than a few months, and you will start building some fundamental expertise in your field.
Fiverr.com is a freelancer website I've started using more over the last couple of months. Freelancers offer “gigs” for $5, including limited services, but then try to upsell into more services for more money.