To get to the App Store on your Apple TV, look in the top menu bar on your Apple TV 4K or Apple TV HD.
Sideload apps: Follow developer instructions to add to your phone. Not common. Risky: Jailbreak an iOS device and load apps to it. This article explains how to get apps that are not in the App Store ...
You can delete apps on your Apple TV if you aren't using them, and then redownload those apps for free later.