Here is a frame picture for reference and also the back panel on a daypack I made. The main attachment points are the top frame sleeve and the ladderloc buckles on the outside edge near the bottom that connect into a frame strap just above the waist belt (they are on the user side of the frame so hard to see in this photo). There also is the lower buckles in the middle of the back panel but honestly I don't think those do much when its on the frame and is more so when there is a load between. Obviously the side compression straps pull the pack into the frame too but it's mainly the slot and ladderloc in my mind that attach the pack.
(Those straps at the top are something I added to run up to the load lifters when the pack is divorced from the frame, not normally used so disregard those.)