How do I show my returns and warranty policy on every product page?

Open Plugins, then Return Settings, and on the Policy / Summary tab turn on Enable Returns / Warranty Summary. Type your policy in Default summary and save. Then go to Appearance → Product Templates, edit your template's Product Details block, tick Returns / Warranty Summary and save. Shoppers then see a Returns & Warranty tab on every product. If one product needs different terms, fill in its own Returns / Warranty summary on that product and it replaces the default.