I wondered about that too. How the songs were composed. Were they written down all the time or just done by memory.
I did some charts for myself when I DJ'd. The song title, the speed, what track, and what to do with it. (Interrupts, Build-A-Bridge to get to the next song, Blend, Turn up Volume as you mix the second song in etc.) that was the only way to do it.
Instead of just throwing songs together I thought of a story line. That is hard. I had the idea of telling a story within the songs.
Then you get Orphans, songs you can't find anything to go with them. You'd think that wouldn't happen, but it does.
Those days are over now. No good parties in Pittsburgh.