This is an accepted solution.
Hi, I currently use a Print on demand app.
If I remove the app, will I lose all my products from my shopify website?
This is dependent on the app, if you did not enter the product information their is a chance as part of the uninstall it will remove any products tied to that app&service.
If the app was the source of the products in the first place be sure to review any legal agreements for compliance.
Do not continue to use any content(text,imagery) you do not possess the copyrights or licenses too.
Your best bet is to export a backup of all your products before proceeding and/or also using a backup app like rewind.io