They don’t use stock Android. They use their software on top just as Samsung does with OneUI. Their software just doesn’t have a name: https://www.androidpolice.com/google-needs-to-give-its-android-skin-a-public-name/

The “custom” chip that they use is just a past generation Exynos. Google designs and adds just a tiny chiplet for machine learning and the like. Otherwise it’s basically a Samsung Exynos chip.

Also, a lot of the hardware components in Pixels are pretty poor. For instance, their batteries are sourced from a Chinese OEM that otherwise sells replacement batteries you’d buy at a knock-off market.

Google does the exact same thing with the Pixels. Except their software isn’t quite as polished or reliable as OneUI. It’s no longer the Nexus line that used stock Android.