Take some Oats off the ground. Cool unprocessed. Mill it a tiny bit to help it cook more easily into steel cut oats. Minimally processed. Grind it down into instant oatmeal - Heavily processes. Add some chemicals and put in a test tube to turn into some weird kind of artificial sweetener? Highly processed.
I try to go as low on the chain as possible, without killing myself. I don't enjoy eating oats right off the ground, or boiled as is (unsure if we can even digest raw oats, never tried). Steel cut I do enjoy - so I stop there.
So back to fruit. If you can get fresh fruit great. If not, frozen is pretty close. If you have to revert to fruit rollups? Probably went too far.
* There is a debate that frozen fruits can actually be better than frush fruit, as fresh fruit is picked unripe and ripened in a truck - whereas frozen fruit can be picked when fully ripe. In this case, the slightly processed fruit may end up being better than the unprocessed fruit. Overall though this is the exception, for 99% of food - the more you do to it the more you lose.
The heuristic of "the less natural the worse it is" is not a very good one.
Potentially a misconception, as well. While modern Fruit Roll-ups are very processed, fruit leather is basically just ground up fruit paste that's been dried. Way more calories by weight than real fruit, but essentially no different.
And that is different from most packaged, canned, boxed, foods which are clearly manufactured as they just wouldn't happen otherwise, in the way that ingredients are mixed and put together with the primary goals of making a product hyper-palatable (it sells), at lower cost and with a long shelf-life (minimizes risks), and only incidentally if at all, with any care for its nutritional value, in which case they throw in whatever fortifications will allow the vendor to claim certain nutritional benefit while not affecting negatively their ability to sell. These tend, as you mention, to just add sugar, fats and salt, in a way that one simply does not find in non-manufactured foods.
As they say, context matters. For example there are many who believe that salt intake by itself is not the problem, instead the optimal ratio of Sodium to Potassium is what becomes problematic. In latter years with all those added salts, Sodium intake has increased, while the lower share of natural foods has also meant a reduced intake of Potassium, throwing us off-balance from what a "normal" body needs.
Er, there's quite a lot of high fat, processed foods in modern supermarkets. Now, they don't put "high fat" on the label, because "high fat" isn't a selling point. but that doesn't change the fact that they are high in fat.
I assume by highly processed, you mean foods altered in such that they are now high in fats, salt, sugars or some combination thereof. I ask, because the term processed gets thrown around (in my experience) for highly preserved foods like canned meat to flash frozen frozen vegetables in the freezer aisle.
The former is obviously worse for you. Taking in high amounts of sugars, bad fats, and possibly salt is detrimental. However, referring to green beans that were grown, picked, flash frozen and delivered as a processed food is somewhat misleading. Flash freezing is altering the food from its natural state. Thus, a frozen vegetable is technically a processed food. The connotative definition, however, implies that more than freezing has happened.