Cost can go up quickly though with requirements. Moving bridges are way more expensive, bridges across active waterways obviously so, cars are another dimension. There is a scandal in my town where a nice retro artsy car-bridge is closed more often than openend and the steel is breaking after 5-10 years. Think millions per year.
I fear that pedestrian bridges are pretty far out of the normal where your from? In my (small) Dutch city I think there are about 500? So they’ll replace at least a few per week (going from 20 year lifespan and the age of most of the bridges). They look like exactly those in the pictures (except the ornaments of course ;).
His city council was working on approving a new housing development that required bridges so they incorporated the designs into their plans prior to construction.
They were built to match the banknote designs.
A completely redundant facade around a fully functional bridge isn't "incorporated into the bridge."
He and his "whole team" (of other artists, probably) were allowed to decorate the bridges that were going to be built anyway.
My home town had bridge failing(no full collapse) and replacement was delivered fast(~1.5 years to remove old one and install new one). A real bridge with 4 lanes(one way) over a river. And then pedestrian bridge over some railway tracks cost like four times and was designed badly from I heard and ended up costing more...
It gets a common design, with everything made to fit together. They can look pretty monotonous. Some people call them living-factories ("woonfabrieken"). The advantage is that they are high-quality homes and all kinds of services are designed in: there's a space for super markets and a few other shops, a handful of restaurants, doctors and dentists, primary schools and daycares. Everything you need for living with a young family.
Infrastructure, like bridges, are also designed in. The overall plan has had an environmental review. Probably multiple, and also other kinds of reviews. It can take years before such a plan gets approval, but in the end the municipal council usually agrees.
There were probably already some basic bridges designed at those spots. If the municipal council liked the idea, it wouldn't have taken much to get the design changed to this.