You can upload designs to create patches, hats, socks, etc... This is somewhat costly and requires a significant amount of money from the start. A cheaper option is to do drop shipping. See Printful.
If you are doing clothing this is the cheaper option. It is a drop-shipping company so you do not have to pay for the products. You simply design them and connect the products to a store website. They ship directly to customers.
Arduino is the OG of the maker universe. This microprocessor can be used for a million different things. Lots of existing code exists out there already.