I publish through Amazon Print on Demand services (Amazon Merch and KDP), and I have a few other stores selling items too. Basically I design the manuscripts and graphics, upload the files to the service, they list it, and when it sells they print and fulfill it and send me a portion of the profit.
One Christmas I made $8k in a month and we went on a cruise with the profit. Those were good times! It’s settled down a bit now as more people have flooded the platforms and copied all the content.