Cereal before milk in most circumstances (the only time I do milk before cereal is if I have unsweetened soya milk, then I will add milk first so I can stir in some sugar so it actually tastes nice)
Honestly I just do whichever one I grab first😭😭. If it is milk then milk goes in first and if it is ceral then cerial goes in first. Just depends which I have first. But I know a bunch og people who would have a hissy fit over be doing whatevre I want lol