How do modern cars regulate temperature so well?

Sorry I didn't read all the posts, but I'll add a possiblity and a little engineering explanation. First, the thermostat (T-stat) in older cars (2015 and earlier?) is what engineers term a "proportional controller". It does not regulate to a precise setpoint, which we term "proportional droop". Carefully test one in a pot of hot water w/ a thermometer and you will see that it starts to crack at say 180 F and doesn't fully open until say 190 F. With a high thermal load (hot day and/or driving uphill fast), more coolant flow is required. That requires the T-stat to open more. That requires a higher temperature. Thus, you see the engine temperature rise slightly. One could decrease that operating band by making the T-stat touchier (more gain), but that risks "negative feedback oscillation".

Most mechanics seem to think that the T-stat is constantly opening and closing. I doubt that. It should settle at a steady opening position (varying w/ thermal load). They must be basing their concept on how a home heater and AC works (on-off mode, termed "bang-bang control" in academic texts not kidding).

BTW, if you want to know if your dash gage is accurate, note the average position the needle runs at and that should match your T-stat's temperature spec (if working perfectly), or measure with an IR gun at the sensor location.

Many of the later cars now have an electronic T-stat to more precisely control the engine temperature, under all loads. I learned that in a recent RockAuto newsletter, so thank Tom Taylor.

Before arguing with me, consider that you will be arguing with many engineering textbooks which millions of engineers in the world understand. Smarter to just google the terms I listed and learn.