Are inserts for the pocket diapers just pre-folds?
No, pre-folds are usually made of cotton, sometimes hemp or bamboo, and are layers sewn together. When a pre-fold is referred to as "premium" it just means it's extra thick, and usually costs a little more than regular but is well worth it. Inserts are usually made of fibers that are specially designed to suck in moisture. In a pocket diaper with microfiber insert the insert will virtually suck "wick" the moisture through the soft liner of the pocket diaper and hold it in the insert. You'd notice when you take the insert out it will weigh a ton, and the liner of the diaper would feel almost dry to the touch. Like I said before though, you can use a pre-fold as an insert or doubler but it just doesn't hold in the moisture or hold as much moisture.