I do a soak in hot water and distilled vinegar for like 10-15 minutes then wash the bottles and it helps get all that off! They look brand new after😊 I think it’s just normal build up, I do it to my bottles/nipples like once a month
Agreed. A vinegar soak will take care of it
Just water marks from limescale I imagine. Try hand drying with a tea towel and should be ok!