When building a world, how much of it do you try to really come up with, vs how much do you have as analogous to the real world?
Depends on the purpose. In the case of the D&D project, Cobrin'Seil, that world is very much a fantasy world, and so instead of trying to reinvent everything from first principles, I just go 'it looks like a fantasy world' and most people more or less come along. There's assumptions people have about how fantastic it is that don't apply, too, but that's its own thing. Like, in Cobrin'seil, the world's origin is unknown and the time stream's been messed with and the gods didn't make the world as much as they found it, with people on it, and don't like considering what that means.
In City of Heroes, my worldbuilding is dealing with a world that kinda looks like if you squint at 2005 America, so there's a lot of things you have to assume happened and came along, like the way that there was an economic depression and there's police violence. In Final Fantasy XIV, my worldbuilding tends to work with 'here's what the official lore is, and the official lore is always stupid and written to the perspective of a person, so let's assume that person was a fool and a racist, and imagine what they might have seen that gave them that idea.'
My general default assumption is to not fuck too hard with the real worldiness of things. Exalted does this, like, in Exalted, things don't fall because gravity exists, they fall because pattern spiders like the pattern of all things falling because one thing fell once, so they just accept and repeat that, to fit with the pattern. That's the kind of thinking that I feel undermines the feelings of the tangible familiar that people can rely on for their play experiences.
A lot of the time what I'm doing is looking at something that has to/does exist like things for people to engage with, and asking 'okay, what do I need to make this exist, and be cool.'