How Did the Artists We Love Actually Write Music?

It’s a really good question and if I had to guess I’d say they probably used all options you mentioned.   I hope you can get some definitive answers from some of the posters here. As far as I know Lennon & McCartney dinked around on the piano for a tune. Whether lyrics came before or after is anybody’s guess. Maybe they wrote lyrics with a tune alongside?   So many possibilities. King Crimson pulled a lot of finished works from jams afaik. In my unprofessional opinion, Most great pop songs depend mostly on great LYRICS BEAT & MELODY. (not necessarily in that order) The rest being icing on the cake.

Edited by Valdez - 14 hours 49 minutes ago at 20:16

Over the years, Neal Morse has provided a few making of documentaries for NMB, Spock's Beard and Transatlantic albums. Each one has at least some material where you see a glimpse of Neal's recording/writing workflow.

There are stories about how a band member wrote a big hit in 5 or 10 minutes on a plane or a tour bus during a sudden wave of inspiration. Sometimes a song just magically pops into their heads, Music, lyrics and all.   

I am not sure what era of the band this applies to, but members of Triumvirat would often have lengthy verbal discussions about their music in the process of creating it, finding that talking about it like that was generally helpful in the process of creation.