The high-bypass turbofan

A pure turbojet throws a small amount of air backwards very fast. A high-bypass turbofan does the opposite: it throws an enormous amount of air backwards fairly slowly, and that turns out to be far more efficient for the speeds airliners actually fly.

Most of the air entering the huge fan at the front never touches the combustion chamber at all — it bypasses the core and goes straight out of the back. On a modern engine, eight or more times as much air bypasses the core as passes through it. The buried jet engine exists mainly to spin the fan.

This is why engines have grown so visibly fat: the fan of a GE9X on the 777X is roughly as wide as the entire fuselage of a Boeing 737. Bigger fan, more bypass, less fuel — and dramatically less noise, because a slow, wide column of air is far quieter than a thin, fast one.