Found it. I've never actually opened the menu item there so I don't think I've every actually seen it.
I poked around Google's AOSP code. I am thinking that the stock OS has proprietary adaptive brightness code that's not in AOSP. Searching around the internet, I read stuff about the phone using "AI" or "machine learning" to "learn" a user's preferences. That's not the case in AOSP. The AOSP implementation does technically live up to the "learn" a user's preferences claim, but it's not too fancy and is very limited.
Kind of what it boils down to is they set up a spline curve, which they call a "short term model". There's a default that's created, then when the user changes the brightness by dragging the slider, a point is added to the model's curve, then the curve is recalculated and smoothed. It doesn't seem to store old points, it just uses the defaults + the new user added point. The phone uses that model until the brightness is changed by the user again, or if the model is invalidated for some reason.