Anything about which I know nothing requires a reasonably detailed explanation, something that will satisfy my own curiosity.
Anything I am familiar with needs to be worked through to the same level.
That's easier. I can look at a passage or a footnote and ask myself whether I would be happy to submit that to our notional examiner who knows nothing about whatever.
So I research, collect and collate detail to the point where I am satisfied with the result and then work the detail into the text while trying to account for the various degrees of detail required by different readers' varying degrees of background knowledge.
Recent trends in education have added an entirely new strand to those tasks.
The first is the poisonous notion that since we don't know what factual information will be relevant in, say, twenty years' time, teachers should avoid a focus on things like facts and dates.
History is not all about facts and dates, but you will never be able to consider cause and effect, for instance, without a sequence of events. The Great Depression, for example, since it happened between them, could not have caused both World Wars. (4)