The factory made fender skirts mount flush and there are NO loose parts, brackets, etc for mounting. The mounting mechanism is built-in on the backside.
There are aftermarket skirts that do not have the built-in clamp mounting mechanism. They have loose brackets and fasteners to attach the skirt to the well opening.
There is another type known as "cruiser skirts" that mount on th outer surface of the fender. They are drastically oversized with respect to the well opening and give the car a radical custom look.