The science of building engines has advanced significantly. Engines are mass produced at nigh higher tolerances these day. If an engineer wants a hole with a diameter of 4 inches today you get a hole with a diameter of 4 inches +/- a few thousandths of an inch. This means pistons of the cheapest mass produced engines today seal up much tighter than those built 40 years ago leading to less exhaust gasses escaping the combustion chamber that reduces efficiency. These tighter tolerances also allow you to run lower viscosity oil which is easier to pump.

Cars are now all fuel injected instead of carburated which means a computer can precisely deliver the exact amount of fuel into an engine, measure how much excess fuel is in the exhaust, how much air is going through the intake and make adjustments depending on throttle position and engine load. Engines also can make adjustments to advance or retard timing of the spark do adjust and when the intake and exhaust valves open in relation to each other. While at low RPMs the air flows slowly through your engine which doesn't promote swirling of the air fuel mixture leading to a less efficient burn. Modern engines can open the intake valve later or open them less to speed up the movement of the air then at higher RPMs it will gradually open the intake valve more and earlier when the air flowing through it is moving faster. The same principle applies to the exhaust side to harness the inertia of the exhaust gasses flowing through your pipes in order to clear out more air from the cylinders through a process called scavenging. At high RPMs you can leave the exhaust valve open during part of the the intake stroke and open the intake valve earlier. This means both valves are open at the same time which allows the inertia of the exhaust gasses to help draw in fresh air through the intake valve into the cylinder allowing the cylinders to have more air and fuel in it. Early muscle cars operated in this mode all the time leading to the distinctive burbling sound at idle which was basically the engine having trouble getting enough air to keep running make it inefficient at low RPMs. Modern cars can adjust valve timing so they operate efficiently at all engine speeds.