I’m planning to sell in the European Union. Is there a clear template for legal policies? I mean, are there any absolute requirements, or can I just write them down using ChatGPT?
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Create/edit your legal policies in Shopify Admin → Settings → Policies (Privacy, Refund, Terms of Service, Shipping, etc.). Shopify provides templates you can generate and then customize. 
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Add links to those policies in your storefront menus (usually the Footer menu) via Online Store → Navigation → Footer menu → Add menu item → Link → Policies. (Shopify policies are linked in checkout automatically, but not always in your storefront unless you add them to a menu.)
By going in Settings → Policies, you’ll have access to auto-generate policies from Shopify templates and settings.
However, you would still need to review them and adjust them based on your own needs and services/products. Plenty of website offer to generate policies by asking you questions on the type of services offered and used (location, tracking, payment providers, etc.).
ChatGPT is also an option, just make sure to validate any output to stay within legality. You could consider using the standard templates and feed it to ChatGPT to have it adjusted based on your own needs.