Is it too much to ask for a stepbystep explanation

I'm not being rude (I hope the title doesn't sound rude), but even though I managed to figure out how to fix the bug easily enough using the hints, I am still a bit bamboozled on how the actual code works to flatten out the dictionary. Anyone here have the patience to go through this with me? It would help me learn a lot if you could.

The code honestly confuses me to no end.

Thank you.