The PikTV device runs on the Android TV platform not android OS that every other device uses. While similar they’re different.
The Core Operating system functionality is the same. Drivers and hardware interface is identical. The difference is the (UI) User interface, is adapted to be used with a controller, not a touch screen.
You only see apps on the playstore that support the android tv platform. Any app can technically run with some coding changes to support it. It’s up to the developers to adopt the changes.
You may find apks (apps) for the android OS platform may or may or work by side loading them. But you can google how to do that.
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AndroidTV playstore apps needs to be specifically built for AndroidTV (as noted supports 10-Ft-UI and remote control), apps lacking support will be filtered out by the playstore on AndroidTV devices.
That said, most apps (unless those built specifically for phones) will run as long as it meets the "min. API level requirement", you just need to use some alternative "playstore" app (try Aptoid TV) or you can pull the apk already installed on an Android tablet using any file browser.