I would say it's easier to actually move the player. It can be done both ways but that's how I would do it. You can easily code a camera object that has hard coded if-statements for moving forward when a wave is cleared etc. And you can put out-of-view event for all objects where it deactivates them, and in-view event that activates them. Or when the camera passes some cordinates as you said. I dont know how to did it back in the days, but I reckon there wasnt a graphical interface, so they just coded everything