Most Roku apps are also on Apple TV. The Roku Channel is one that isn’t, so far. For that, you would just go to the TV’s app mode and choose that instead.
Also, if your are NOT subscribed through Apple but DIRECT with the services, then you can use the Roku apps and just log in. Your info will work fine. I have all 4 major devices (Roku, Fire TV, Google TV, Apple TV), and can easily switch between them. Each has plusses and minuses in app support and features.