According to them, Vekoma does build the coasters. They are a full-service manufacturer, they're responsible for both the design and the construction. Parks may hire other companies for specific parts of the project, like landscaping or additional theming, but Vekoma claims they handle the actual construction and installation of the ride.
(Sources on Vekoma's website)
https://www.vekoma.com/about-us
https://www.vekoma.com/company-profile