So I vaguely know cards like Magmatic Chasm still affect creatures that enter the battlefield after they resolve (or, e.g. lose flying after resolution) because they sort of… affect the rules themselves instead of the creatures? But why is that exactly, and how do you tell cards like that apart?
The distinction lies in whether or not a spell or ability's effect modifies the characteristics or changes the control of a card or permanent.
109.3. An object’s characteristics are name, mana cost, color, color indicator, card type, subtype, supertype, rules text, abilities, power, toughness, loyalty, hand modifier, and life modifier.
If a resolving spell or ability does change characteristics or control of one or more cards or permanents, then the set of objects it applies to is determined and locked in when it resolves. It will continue to apply to those objects and only those objects for its duration.
If a resolving spell or ability doesn't change the characteristics or control of any object, then it's "setting up a rule" and that rule will apply to all relevant objects when it's checked.
For example, compare the two cards above: Dirge of Dread grants fear, which is an ability, to all creatures until end of turn. This is modifying the characteristics of those creatures by adding an ability and so it applies only to the creatures that are on the battlefield when it resolves. Dread Charge, on the other hand, doesn't grant an ability but rather just sets up a rule that's true for the turn. Since it's not modifying control or characteristics, it applies for the whole turn and a creature that enters later in the turn will still be affected by it. Also a creature that stops being black will no longer benefit from it.
