This is a topic of conversation that would truly start arguments in a walkers café or pub. What should be in your rucksack is very personal to you. I for instance don't carry lots of warm clothes because I typically am hot, my wife on the other hand carries extra warm clothes with her as she gets cold quicker than me.
It also depends on where you are walking. Do you need a windproof layer because we are walking across the tops of the fells, will you be in the shade of a valley so need a warmer layer. Have you checked the weather forecast? Was the weather forecast a normal forecast or was it written specifically for the mountains?
So what do I carry with me?
In my rucksack
For a one day walk I would carry a 35-45 litre rucksack
Food and drink
Snacks (I get hangry!)
Small first aid kit
Headtorch (just in case I get caught out, it's happened many times!)
Any medication that I need
Emergency shelter (sometimes called bothy bags, bivi bags, survival shelters, KISU shelter)
Survival Bag (these can be a big plastic body bag, which doesn't bode well for your survival situation, but I use a Blizzard 3 layer survival bag https://www.blizzardsurvival.com/shop/blizzard-3-layer-survival-bag/
If the weather will be nice and sunny
If the weather looks wet and cold
In my pockets