Ash here from the Shopify team. Welcome to the Shopify Community!
It's awesome to hear that you're interested in working with Shopify to create and sell products via Print on Demand. You ask a great question! While t shirts are absolutely a popular staple when it comes to Print on Demand, you're not limited to only those! Many Print on Demand services offer a wide variety of products you can print your designs onto, including those found in the Shopify App Store. Here are a few popular options for you to check out:
Most Print on Demand applications are free to install, but require payment once you start using the app/making sales. Before making a final decision, I suggest looking into some of the options that are available, and double checking with the developers of each app to ensure they can accommodate your production needs. I hope this helps, and feel free to reach back out any time if you need further assistance, or have any more questions!