Sideloading is of particular interest to get foreign TV apps and also other Web Browsers onto your Android TV.
1. Preparing the Android TV
Just like on your Android devices you'll need to enable Other Sources in Settings/Security & Restrictions.
To get an icon to start your sideloaded apps you will need to install either the Sideload Launcher or the HALauncher (my preference as it enables making shortcuts).
2. Sources of apk:s
Many apps on the Google Play Store are not available on Android TV, but many of them do work. Other sources are Amazon Appstore, AndroidAPKsBox, APKMirror, AptoideTV and Galaxy Store to mention a few. Some of these have an Android app whereas others can be accessed via their Web Site. AptoideTV is even available on the Play Store for Android TV, and some of its app you can be installed directly - no need to sideload.
3. Downloading apk:s
You can download the apk:s to your PC. I usually install the app onto my Android Tablet, and use a file Manager to backup the apk, such as ASTRO File Manager (App Manager) or ES File Explorer (Library/APP).
4. Transfering the apk:s to the Android TV
4.1 Copying via a USB Flash Drive
4.2 Copying via SMB Shares
4.3 Copying via a Cloud Server such as Dropbox, Google Drive etc
4.4 Copying via an Android App such as AirDroid, Air-Share/Launch, PushBullet, FastFile Transfer
4.5 Copying via an FTP Server such as FTP Server on Android TV
5. Installing using a File Managers
I prefer using X-plore to access the apk:s on the Flash Drive or on a Cloud Server. Also ASTRO (sideloaded) would do the job - but I have had problems with the ES File Explorer.
AirDroid installs the apk directly, and Air-Share/Launch even bypasses the apk by installing directly from your Android device's installation. I have had some problems with PushBullet and especially with FastFile Transfer.
6. Shortcuts on the HomePage
You can add Web Shortcuts to the Favourites bar on the Home page by using the TV App Repo.