Z-wave requires certification[1] which includes alliance membership and compliance testing, and there you can do your own stuff using shields or modules[2].

Some manufacturers also make Z-wave devices, like this[3], which has some generic low-voltage inputs and outputs.

So while not as trivial as other things, it's possible to do a lot of DIY stuff.

[1]: https://z-wavealliance.org/development-process-overview-2/

[2]: https://z-uno.z-wave.me/module/

[3]: https://www.fibaro.com/en/products/smart-implant/