I don't think there can be a hard-and-fast rule about when a question is about an individual character, when it is about plot events and when it is about worldbuilding. At some point, it comes down to judgement calls about specific questions.
A good rule of thumb would probably be:
- if the answer will affect all characters in your world equally (possibly restricted to all characters of a given species only, based on the capabilities of that species), then the question is likely about worldbuilding and thus on topic for us
- if the answer will affect different characters in different ways, or the answer will be different for different characters, then the question is likely either about plot or (more likely) individual characters and thus off topic for us
Note that as described in the help center, the "world" does not have to be the whole planet or the whole universe:
A setting might not be a planet; it can be larger than a multiverse or smaller than a village.
The reasoning for the bullet points above is that when you are dealing with individual characters, or plot, what happens to one character will be different from what happens to another character or how another character perceives those same events. However, when you are dealing with worldbuilding, everyone who perceives the element or event will perceive it in essentially the same way.
- A group of charging unicorns would appear basically the same to everyone, and would be (largely) the same thing regardless of whether you are in front of or behind them, given that the unicorns remain the same. The capabilities of the unicorns do not change.
- How a particular individual reacts to those unicorns charging toward them would be different for every character.
Hence, the former is about worldbuilding, and the latter is about individual characters' actions.