I crated this gist once when I was trying to understand how coroutines are compiled myself, perhaps it helps you 🙂
https://gist.github.com/fxshlein/49704a5999fef8721a454bb5255b28ea
I cleaned up coroutines code I decompiled using the decompiler in IntelliJ (so also Fernflower), and turned it into standalone java code that actually works, and can be pasted into a scratch file in IntelliJ, and you can run/debug it.
Its mostly uncommented, and I don't know how accurate it is these days, but perhaps it might still be useful