How do modern cars regulate temperature so well?

One of the ways modern engines run consistent temps and warm up fast is newer thermostat designs.

They have a second "plug" which controls the bypass. When closed the coolant circulates in the block only, when the Stat opens the radiator comes into play.

Then add on massive flowing electric fans, well sealed core supports, under Chassis aerodynamics which helps evacuate air front-to-rear to prevent hot air being recirculated from the engine bay back to the grille.

Of course not having a big rowdy cam, lots of waste heat, imbalanced cylinder filling, and headers also helps..