Engineer working in the automotive industry here.
I’d say mainly 2 things. Very fine control due to extremely powerfull ECUs and very tight manufacturing tolerances being economically achievable for mass production.
A modern ECU had immense control over all functions of an internal combustion engine, and can fine tune parameters litterally mid-combustion. The programms running in the ECU are very complex, can affect every part of the engine. And all those adjustmens happen in litteral miliseconds.
Let me talk with an example. In the mid-late 90s you started having ECU controlled port fuel injection. Most things until then if they needed some type of control, it was done mechanically. With cables, linkages, vacumm lines etc etc. There is not that much processing needed to control a 4 port injector. 10-15 sensors maybe.
Now go the modern ICE engine. You have… high pressure direct injection with piezoelectric injectors (allows multiple injections per cycle with fine control), electronically controll wastegate in the turbo, variable valve timing and lift. Oh and probably close to 300 sensors all around feeding data to the ECU. Literraly everything that moves, vibrated, heats or pressurizes, I can guarantee you it has a sensor that reads that. Plus a very complex SW that uses all these data to make adjustments in everything.
As for the manufacturing tolerances. Valves, piston rings, gaskets, turbine impellers/bearings. All things mechanical that move are manufactured more precise than ever. Also oils (and chemicals in general) are more advanced that ever. Today’s oil is nothing like the engine oil from 15 years ago.
Yeah the modern car is literally a computer on wheels.
We try to account and deal with as many failures as possible without crashing the whole SW.
Lets say you lose one sensor. You can probably still operate in an acceptable manner by guessing what is your missing value by extrapolating/calculating data from other sensors. Or by using pre-defined data for such cases.
There is a lot of redundancy built in the software to deal with multiple hardware failures. And thats the majority of the cost of development is in testing & validation.