Roku itself is a walled garden. You cannot develop apps for them unless you are blessed by them. Last I checked the SDK was closed (as in, closed to even try out developing for Roku).
The BrightScript API for Roku app development is freely available, and any Roku set-top box or Roku TV can be put in developer mode where you can sideload your own channel. You can also upload private channels where users can install your content via a weblink -- they have to go through certification to be in the on-device Roku Channel Store. All developer docs and samplers are all on GitHub, see https://github.com/rokudev