I didn't vote either, because I am not sure. I think I start with some musical idea normally, but never with a completed tune.
With me, I'm not sure if a song is ever really finished - it just moves to the occasional polishing stage
So, what happens is I am maybe working on one song. I go over it and over it, changing the music and changing the lyrics until I am fairly happy. In doing this, I get to a point where I can't really hear it any more so I play something else. This may be an old song, which can still change. I may try it in a slightly different style or mess around with the words - then keep any improvements. Then I will just mess around.
Out of that "messing around" comes something new. Sometimes this is a riff or a change that I like, sometimes it has a lyric attached. Sometimes it's an old lyric that didn't fit a particular song, but that had something about it. This new thing might become the seedling for a new song.
I will normally have a few "bits" I am working on. A main song. Maybe a tune. Maybe a lyric looking for the right tune. Very, very occasionally a complete song.
I work on these and throw bits away, chiseling and polishing. By the end, I am not always sure where they started because they may have changed a lot - and could still change.
Sometimes I just tire of them and they get dumped. As someone on another forum once said "If you can't listen to your own song 1000 times, why would 1000 people want to hear it once!".
It wasn't always this way. I'm sure I used to be able to simply toss songs out quickly. Maybe they were no good, maybe they were OK. Now, however, I find it harder. And I still don't know if they are any good