I've actuallly sold more books based on what I worte on Medium than I have with paid ads.

That is great that you have been able to sell so many books without ever advertrising them. That is great that you have been able to find a loyal readership based on your writing alone. I've been writing and publishing for 24 years now, mostly fiction, but so far it has paid much. As is the case with about 98% of writers. Some of it just comes down to luck.