It's not. And any effort to do this at scale would be quickly shut down by physics and economics, not the government. Modern computing technology is a wildly complex supply chain, with extremely specialized manufacturing equipment and facilities. Billion dollar fabs are worth billions of dollars for a reason, and it's not for the real estate or the views.

Trying to determine the best "diy chip" sounds like a fun project and an admirable goal, but if you actually wanted something useful I'd wager you'd be better off buying esp32's in bulk so you'd have all the spares you might need.

Anything that's got wireless/bluetooth integrated is probably backdoored already, but the esp32 certainly has been (https://www.techspot.com/news/107073-researchers-uncover-hid...)

The entire point of of designing your own chip is so that you know there won't be any surprises. Nothing undocumented.