Often, self-employed side hustlers don't think about their side hustle as a "business." A business connotes formality when side hustles are often quite casual.
If you file a Schedule C with your tax return, you have a business. You are considered a sole proprietor if you run your business on your own, with no partners or employees. That is the type of business owner I have in mind for these trainings.
If you're not sure if you are supposed to file a Schedule C for your side hustle income, review the definition of sole proprietor available from The Balance. If you're earning money regularly but don't have an employer (instead, you likely have a number of clients or at least the option to take on several clients), chances. are very good that you're self-employed and should file a Schedule C.
Any business that turns a profit can benefit from learning the "Best Practices for Your Self-Employment Side Hustle."