EDIT: As of the wnter update, it seems it no longer works the same way.
From the patchnotes:
Cities can no longer receive yields from more than one regional building per type; they take the highest (ex. production from multiple Factories)
Cities can no longer receive amenities from more than one regional building per type; they take the highest (ex. amenities from multiple Stadiums)
I came back home and tested it, it just so happens that I have a perfect example to answer this in my current game.
TL;DR : The bonus extends from the District itself, not the city center.
As you can see here, both Kwila and Kwango have industrial districts. By hovering over the city's production, I can see that Kwila receives a +6 from factory while Kwango only receives +3 from factory.
Both cities are 7 tiles from each other, and no other cities or districts of my own are within range of Kwila, so we must assume that Kwila is getting its +3 bonus from Kwango's industrial district, which is exactly 6 tiles away. Meanwhile, Kwango does NOT receive a +3 bonus from Kwila's factory, which is in the industrial district 7 tiles away (which is poorly placed, in hindsight), like the city center.
I will also assume this will work the same way with wonders, though I have no way to back it up right now. It also makes sense IMO, as it makes positioning of your districts and wonders a lot more important.