I like minimalist type bags as I hate the dreaded rattle.
Bar bags are good about 3 litres. Used for food, warm layer, pump, spares, tools, gels, glasses
Bum bags are good about 1 litre, used for keys, phone, snack, windproof gilet/jacket, essential spares, money
Saddle bags are best less than 0.5 litre, used for tools or spares
Small top tube bags are good for food
A bottle feed bag is also good to add extra food or water bottle
Bargain bags can be bought from Decathlon, Halfords, Planet X, eBay and Amazon.
Or a tool bottle is awesome on the cross/gravel/MTB marathon bikes.