80% building. I do specialize in this field of building things quickly and sustainably.
And in a lot of cases, like a "UI upgrade" of a decade old project we had lately, the hardware, the API, and the tech are so different that it's easier to rewrite from scratch.
Some large companies also contract other massive companies to build things for them, where the code belongs to the contractor. Updates can be very expensive, and so people often rebuild.