sold older model

That would depend on the exact model of Mac you bought. In 2020 the available iMacs for sale where the 2020 27" Retina 5K iMac, the 2019 21.5" Retina 4K iMac and the 2017 21.5" non retina entry level iMac.

If you chose the cheap entry level 21.5" model, then you got an iMac with hardware specs that first debuted in 2017. The iMac itself would have been a brand new one manufactured in 2020, but still labeled as a 2017 model since those where its specs.

Just like Apple sells the 2020 27" iMac model currently, since it has not released a new update for the 27" version since then.

That model, that appears to be the crippled educational/institutional model with a sluggish dual-core processor, was in the line continuously—and unchanged—for over four years. It was made to sell US$200 cheaper than that standard base models. This is from the MacTracker database for that model:

If you want to see when it was manufactured, there are serial number lookup web sites that return the week of manufacture.

Example: https://everymac.com/ultimate-mac-lookup/

I suspect yours will show a 2019/2020 manufacture date.

These are not like cars.

Because that's the model you chose to purchase. They were still being sold as the entry level option in 2020.