Just read the Jon Savage book “The searing light, the sun...” recommended.
During rehearsal when writing songs, sometimes Sumner and Hook
would have Stephen play a ‘drum riff’ a repeated pattern to inspire them to come up with something. Stephen was interested in that mechanical like repetition, melodies on the toms.
Amongst themselves they never talked about where the songs came from. An amazing band, each member was so integral to the sound of Joy Division, coming together, truly magical.