Nearly complete engine models. Put in the most critical parts, like the turbopumps & injector plate. Might throw in a few more critical parts, but you can't see anything in the complete ship or maybe you can if the camera is directly behind the rocket.

Every time you add a part, the need for another part becomes clear.  There was an idea to make just an engine test stand simulator with a very detailed model, but it would get boring.

Still hard to believe that speech actually happened or they still intend to carry a 2nd set of engines just to get a different expansion ratio.

The lion kingdom discovered the kerbal verson.

https://spacedock.info/mod/1477/Kerbalized

There's a lot more detail, but it's all very cartoonized.