"those are ON FOOT boundaries. You basically run out of "walking" area. so you just get on your ship and go to the area next to it (on a ship or by fast travelling), and continue your journey from there. Planets are divided into areas, basically. A planet is an alloted set of areas instead of a giant ball full of nothing or procedurally generated junk like on NMS. So yea they didn't built a thousand planets by hand, but more like 1000 sets of areas."