Does this game account for relative motion? For instance, if an effect depends on someone entering an area, what should that motion be relative to for it to count?
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I'm not entirely certain what you are asking: Generally speaking, areas don't move, so relative movement is irrelevant.
For instance, spells like School Spirit read like "Enemies who begin their turn within the effect or who enter it must succeed at a Will saving throw or be shaken for 1 minute" and play exactly as written: The spell has a distinct non-mobile area (in this case, a 30' spread within close range of the caster at time of casting) and foes need to make a save against its effect whenever they start your turn in this area or you enter this area after the start of their turn. If the area of effect does somehow move, then any foes in the new area would presumably now fall into the 'Enemies who begin their turn within the effect' category.
Could you provide an example of when you think relative motion might come into play?
Precisely that. If the area moved to the enemies, then the enemies entered relative to the area, but not relative to the ground. Likewise, if the enemies are on a moving platform that brings them into the area, they are entering relative to the area and to the rest of the ground, but not relative to the platform. Which matters more? And if the spell is cast by someone on the moving platform, does it stay still? Relative to what? The moving platform? The rest of the ground? The caster?
PF2e doesn't have rules for this either. I thought SF might because space travel.