Aug 3rd '19, 02:48
It doesn't. The whole level is read and runs at once, not doing so would break the functionality of many, many maps. Some maps will refrain from activating monsters until you get near them, however, this is not a mandatory feature, especially with A1's increased limits. It really doesn't use much memory to do all that, this was designed in 1994 after all.
Maybe you're thinking of only rendering what's in sight? In which case, I don't know.
Or are you thinking of loading textures and sprites? In which case, it loads any required collections for the whole level at the start. However, since it loads on a collection basis, that means it does load a lot of unnecessary textures and sprites, which is a problem in some scenarios for loading times and 32 bit Windows builds.