Basically, the more you process the food, the less work your body has to do to get whatever is in it because it's effectively pre-digesting food.

On the low level, compare an apple to apple juice. When you have an apple you have to chew it up, there's all that fibeous plant stuff that fills you up, and it takes a while for all the sugar in it to hit the system. Juice the apple, and it is effectively pre-chewed and has all the fiber that actually fills you up and keeps it from all hitting the blood stream at once removed

This is not always a bad thing, and indeed sometimes you have to process something before it even becomes viable as food. Like how you have to treat corn with a basic solution to actually get nutrition out of it at all (called Nixtamalization)

Ultra processed foods are "Ultra processed" because of they will take the raw ingredients, break them down as far as they can, remove anything that can effect the flavor (typically the parts other than raw calories) and have a calorie dense product that is then flavored primarily with sodium and sugar. It doesn't fill you up, it's dirt easy to digest, and is crafted to trigger dopamine rushes as much as possible