How Modular are your Modular factories?

Modular factories to Mega factories are a spectrum. I prefer modular factories, but how modular do you guys get?

So typically, for example, a HMF factory, I would make everything from scratch on site.

But I've also been thinking about the logistics of making like a giant iron factory with all the iron in the area, make the basic iron products on site at the iron factory, and then ship it to the HMF factory.

Like, I also just saw the post of someone's copper sheet factory, where presumably they're making copper sheets en masse and sending them where they need them.

Does this make actual logistical sense? I feel like the cost benefit might be strange. Maybe it's good in the late game? Idk what do you guys think?