Jeff B. answered 08/15/24
Live Online Songwriting, Composition, and Arranging Lessons
You will generally find that the songwriting process is largely done by one individual in the band (sometimes two if they're a songwriting team).
Rarely does the songwriting process take place in the rehearsal room. The songwriter(s) will bring in a number with it more or less fleshed out in their mind what the others musicians' roles are and work it up to performance level from there, working out any bugs and putting together the final arrangement.