When my iPhone was manufactured

If you have a Mac or access to one, coconutBattery will give an approximate manufacturing date. It extracts it from the serial number.

https://www.coconut-flavour.com/coconutbattery/

You can check the warranty here:

Check Your Service and Support Coverage - Apple Support

While that would only give you information on the original date of sale, it's probably more important than the date of manufacture

Why is the date of manufacture relevant to anything?