Not an expert but I think ultra processed food has two main aspects, one is additives and preservatives. And the other is our body doesn’t need to process it as much to digest it. If you eta rice/bread your body has to break that carbohydrates into glucose which takes energy. Now if you directly take suger/glucose then eating the same calories would be a lot more plus calories since your body doesn’t need to work hard to process it. Furthermore it has more pure calories per same weight, so you end up consuming more to feel full compared to eating something not as calories dense.
Again I’m not an expert on this. But the problem is think comes from sharp change in the type of food within few generations. Since we have started cooking food more and more we have gotten weaker jaw and bad teeth with results. But something that happened over a long time, vs something that happened within last 100 years has a different health impact.