I think we can all pretty much agree that it is bad for us...but why? What part of frying food is specifically bad for us?

With some foods there is a fried batter on the outside of the food after it is fried, like with fried chicken. To me that is more understandably bad for us. But is it the flour, or whatever else, that turns into that fried layer on the outside of the chicken...is that what is bad for us? Is it the oil that it is fried in that is bad for us?

Then we have items fried like French fries where they are submerged in oil but leaving no such fried batter on it. The first option listed above doesn't exist here, so what happens that makes it bad for us?

From my understanding most cooking oils aren't good for us and that could very well be why fried food isn't good for us. But there are some cooking oils that not only are said to be ok, but they are said to be good for us...like olive oil, avocado oil, and some others. It is advised to use these cooking oils when cooking, so would it be ok to fry food with those cooking oils?

I am on a limited diet and looking for ways to expand on what I am eating. Plus I am generally trying to get as good of an understanding of health and diet as possible. Any help here would be greatly appreciated.