Avoid any foods that involve multiple rounds of processing, a term that includes baking, frying, adding preservatives,sugars or oils. Generally, if it has a lot of sugar or oil and has a weirdly long shelf life, be suspicious.
Drift towards: easily washable (smooth/peelable) fruits and vegetables, 100% whole wheat bread products, simple meat products like whole animal parts or deboned animal parts.
Dairy lives in the middle ground. If you have zero lactose problem, most dairy is mostly okay, just watch for sugar levels and recognize that most dairy products are calorie dense. Nuts are in this group too for the same reason but oil instead of sugar.
Bonus points for consuming real pro and pre biotics when you can. In the United States this is pretty limited to live culture yogurts, refrigerated kimchi, and refrigerated sourkraut.
For popcorn at least, I'd assume it's the prepackaged microwavable popcorn that gets flagged as UPF, where it's encased in hydrogenated oils, salt, and preservatives. It's hard to think that popcorn you make at home could be considered UPF, considering that it's literally one ingredient with heat applied to it (and oil I guess if you're popping it on the stovetop rather than an air fryer).