When you actually look in to the details of papers on this topic you'll find that it's actually not defined consistently across the literature.
Ultra processed foods are foods made with industrial chemical ingredients either extracted from food or synthesized.
What are industrial chemical ingredients? For example, is sodium hydrogen carbonate one? It's a chemical, produced industrially. Is food made with it in a shop different to food made with baking soda at home? Why?
When you dig into the reason why any specific "ultra processed" food is bad it boils down to:
- not enough fiber
- not enough vitamins and minerals
- too much salt
- too much sugar
- too much animal fat
And if you control for those things (which we have known for a long time and are standard nutritional guidelines) then the effects of food processing on health outcomes basically disappear.