Where my phone was purchased
How do I find out what carrier I was with when I originally purchased my iphone?
iPhone 6s, iOS 14
Question marked as
Take out and look at the SIM,
Or locate the original electronic purchase email.
If you have neither then it will be trial and error.
You may be able reach out to Apple Support directly to find out, but Apple cannot unlock it. Depending on your region, you may be able to reach them here: contact Apple Support