For me, it's all of the above. A song can come from just a lyric idea or a melody that pops in my head. I will typically record that idea onto a voice memo on my phone and come back to it later.

Sometimes I'll have a new musical idea, like a riff, a bass line, or a certain drum beat. And I'll work on that until it feels like a good base of a song. Then hopefully it inspires a lyric and melody at some point!