How do I determine when my iMac was built?

The easiest method is to click the Apple symbol in the upper left of the display and then click :About this Mac” and it will tell you the model year.

How to find your Mac's model and age? - Apple Community

For Mac serial numbers prior to about 2021, the week of manufacture can usually also be decoded from the serial number:

https://forums.macrumors.com/threads/decoding-apple-serials-where-when-hardware-was-assembled-1983-2021-and-apple-model-numbers-1977-present.2310423/

Yup, I would agree with this method (the one posted by rkaufmann87).

Find the model name from back and put it into Google. You will know details about the device.