MTB'r,
Welcome and congrats on looking at a pop up!!!
Will try to help explain a few different terminology to help you.
It sounds like the truck has a "receiver" hitch mounted to the frame. You the slide a "ball mount" into it and secure the ball mount with a pin. The correct size "ball" needs to be securely mounted to prevent any issues.
Depending on how tall your truck sits, and the height of the pop up "coupler" (part that attaches to the "ball") when level, you may need to buy a different ball mount that has more drop or less drop: meaning the distance the ball mount places the top of the ball from the ground. The different ball mounts (usually referred to as "drop") allow you to get the trailer level or close to level.
Etrailer has a ton of
videos explaining towing related items. Also, do a search for a video that explains how to choose the correct ball mount. Didn't see it on Etrailer, but probably missed it.
Most states have a trailer brake law based of total trailer weight. Some require brakes for over 2k lbs, others as high as 3k lbs or 3500lbs. Here in Mi it is 3500lbs (if I remember correctly!!!).
Remember that when towing a trailer, the trailer doesn't follow the truck exactly when turning. Like the trucks rear wheels being closer to the curb than the front wheels are, the trailer wheels will be even closer than the rear wheels. So make sure to take turns a little wider than you normally would so you don't run the tires over the curb.
And speaking of tires, check the tires age and condition also!!!! If in doubt, replace them as it is suggested they have a 3-5yr life span, mainly due to a lot of sitting (non use).
When you first get the trailer, go to an empty parking lot, set some orange cones out and practice turning without hitting the cones. And practice backing up as well. Shorter trailer are harder to back up due to the wheel base. But a longer ball mount helps make it easier. The ball mount length is measured from the pin hole to the ball hole.
Looking at pop ups that age, make sure there isn't any water damage. If the damage or wood rot is to bad, the bed ends (part that slide out) can literally fall to the ground. So maybe try to find someone familiar with checking pop ups out to help you.
Good luck, and continue to ask any and all questions!!!!