Enormous chunks of land surrounding the three airports have actual zoning.
The rest has de facto zoning. It just isn't called zoning. This is why Houston looks pretty much like any other city with zoning.
A lot of what can and can't be built is covered by deed restrictions, which are enforced by the city. (i.e., a developer may put a restriction in the deed stating that only residences can be built, and no commercial activities.)
In addition, there are gazillions of 'land-use' ordinances that control the height of buildings, lot size, parking, etc.
While it would be kinda neat -- as an experiment -- to see how a city would develop without zoning, Houston sure as heck isn't that example.
https://kinder.rice.edu/2015/09/08/forget-what-youve-heard-h...
[edited to supply link]