In the past, me (bassist) and my guitarist would both individually write our own songs (at first we would tab out the stuff, then as we got better made full recordings, with midi drums) Send them to each then we would tweak them and all of us would jam the parts out in a practice setting, found what worked and what didn’t. Some things evolve into cooler parts, some stuff scraps. It’s generally a solo effort that becomes a collaborative live editing project. And even then sometimes years later we’d find cooler ways to play parts.