Also, I have a sort of dumb and naïve question that I have always wanted answered. How do bands know what to play if they are learning a new song? I assume that, say Patrick Stump or Ray Toro or whoever don't hand out sheet music to everyone. And even if they know the chords and melody, how do they know the rhythm and when to do things? I have only ever played French Horn, so I have always gotten my music handed to me
That's a great question, Sophia!
There's probably as many different ways to do this as there are bands, but there's a couple of main ways that bands will write new songs.
First is the jam session, where somebody will start something... A rhythm or a melody line, or set of chord changes, and then others will join in. This is a loose, improvisational way of finding songs. Ideas and grooves will come out of a jam session that can then be turned into a song, with some work.
Sometimes a band member will write a lick, or piece of music, and show it to the band, with ideas of what he or she thinks will go with it. Then the rest of the band adds stuff and see what works.
Sometimes lyrics will come after songs, and in other bands, the lyricist is the driving force. The lyrics will come first, possibly with a melody, possibly not, and the song is crafted around that.
Sometimes the lyricist works with the drummer (since words and rhythm are so closely related), or sometimes the lyricist *is* the drummer, and the rhythm and meter of the piece is set first, with the music growing on after.
Sometimes the band members are actually highly trained musicians, and will prepare sheet music.
The expectations also vary wildly depending on the style of music being played. In garage bands, you probably do most of your work in jam sessions. If you are a violin player joining some avant garde string quartet (Kronos Quartet, say), you'll be expected to not only be able to read and play sheet music on sight, you'll be expected to write your own quartet pieces to contribute to the group.
As with all things, I'm sure there are situations where those expectations are reversed.