As @Strega mentioned, Roku apps are called "channels", and there are 1000s available in the channel store. Most of the "private" channels are specialty (limited interest) or perhaps porn. Some private channels offer copyrighted content illegally, and Roku remotes them without notice once discovered.
Roku devices don't actually have any channels pre-loaded. When you create a Roku user account and link your first device, Roku will load some common channels that the majority might use, such as Netflix or Hulu. But you can remove any of those if you don't plan on using them, so you're not stuck with them taking up space.
One thing you cannot do on a Roku is to "sideload" an app like you can with an Android device. If the content you want isn't available in the channel store, and no one has created a private channel, there's no way to take an app from somewhere else and install it. Roku uses a modified version of Linux, not Android as the underlying system, so there's nothing that can be downloaded from the Internet and installed.
But as mentioned, there are literally 1000s of channels available in the Roku store. Browse the link provided above and you can see what's available. One channel not yet available is HBO Max, as the two companies are still trying to come to an agreement.
Dan
Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.
