A nutritionist will be better able to explain this but I’ll give it a try :)

You’re maybe overthinking the sugar part of the equation. Berries/fruits contain natural sugar that is a part of the fruit itself. Your body processes that differently since that sugar comes integrated with other nutrients (fiber, Vitamin C, antioxidants, etc.). And you typically won’t want to eat say a few buckets of berries in one sitting to equal the same sugar high you get in a processed sugar, all that fiber will feel much heavier and your body is just going to tell you to slow down on its own.

The much worse types of sugars are added sugars e.g. sugars that were processed and now exist separately, then re-added into something else. Take your berry example, process all the sugar out of them so only the sugar exists, then you add those sugars to some other food you wanted to sweeten. Now your body won’t absorb that as a natural sugar complete with the other nutrients the berry used to have… you’re only getting the bad without anything useful. So you can gobble a whole ton more of those added sugars to get your sugar high without your body getting any indicators to hey, slow down, maybe it’s time to stop eating these added sugars.