The Google Play Store on Android TV is a slimmed-down version of the smartphone version. Some apps are not Android TV-compatible, so there aren't as many to choose from. However, the operating system is capable of running any Android app, making sideloading apps on Android TV a popular activity.
But there's an issue. Apps that you sideload will not show up on your device's home screen or list of apps. The only native way to access them is via the Settings menu. Thus, the solution is to install a third-party app. Sideload Launcher is the easiest one to use. When you open it, you'll see a list of all your sideloaded apps.
Download: Sideload Launcher (Free)