I think , just going from what they’d said in interviews etc, that it varies. I know Tre has said that sometimes Billie comes in with the song almost fully formed and his work is largely done for him due to Billie being such a good drummer himself but we also know they have brought things, e.g, famously Longview bass line and Billie has gone from there. I can hardly believe there could be a hard and fast rule, more like a general trend.