The first time I saw a jet engine opened up, in the hangar at our Air Force technical school, the strange part was the back.
It was an old Westinghouse turbojet; I believe a J34, though I am not certain of the exact model. I had always pictured a huge burner at the back, the flame you see behind a fighter. The back of the engine was empty.
The fire is in the middle
A jet engine burns its fuel in the middle, not at the back. Air comes in at the front and is squeezed by rows of compressor blades. Fuel is burned in combustion chambers that sit between the compressor and the turbine. The hot gas drives the turbine, which drives the compressor, and then leaves through the exhaust.
The back is that exhaust: a pipe. On engines with an afterburner, the section that makes the famous flame is also mostly an empty tube, with fuel sprayed into it only when the pilot asks for it. What looks like the engine from behind is the part with the least in it.
The front, and a line on the outside
The front was the opposite: blade after blade, row after row. On the outside of the casing there was a line marking where the main stage sat inside, so you knew where it was without opening anything.
A ring built to be cut into
Around the main blades there was a ring with a honeycomb shape.
It is there on purpose. The tips of the blades run very close to the casing, because every gap there is air leaking past and power lost. But a hot engine grows. If the blades expanded slightly with the heat and touched a solid wall, they could tear the engine apart.
So the wall is made to give way. The blades cut into the honeycomb instead of into anything that matters, the engine survives the flight, and the ring is fixed after landing. It is called an abradable seal: a part designed to be damaged, so that the parts around it are not.
Metal grown in one direction
The detail I remember best is about the blades themselves. We learned that they are made so the grains of the metal all point the same way: outwards, along the radius, in the direction the centrifugal force pulls.
A spinning blade is being pulled outwards the whole time it runs, while it is very hot. In ordinary metal, the boundaries between grains are where it gives way under that kind of load. Cast the blade so the grains run along its length, and those boundaries no longer cross the direction it is being pulled. This is called directional solidification.
None of that is visible. A blade looks like a blade. The engineering is in how the metal was allowed to freeze.