Google ATV Play Store - To download apps that are AndroidTV compatible and can show a banner icon on the homescreen launcher.
The Google ATV Play Store has a lot less apps compared to the APKPure app store or the Google Play Store found on a phone.
APKPure App Store - To download normal Android apps compatible with phones, tablets etc.
These apps won’t show a banner icon on the homescreen, so you have to access them through the AppDrawer app shortcut on the homescreen that can show their icons.
This app store can also keep non AndroidTV apps updated since the Google ATV Store only keeps AndroidTV apps updated.
You can also use Aptoide or UptoDown app stores to install more apps.
Netflix and Amazon Prime both require separate licenses that cost in the millions to be able to use the AndroidTV compatible apps. They also require locked devices, so any open-source ARM board that works with custom firmwares or can boot Linux can’t get licenses from them.
So APKPure or sideloading the phone versions of these apps still works.
Google blocks Chromecast on non bootloader-locked devices and devices not certified by them.
You aren’t allowed to boot Linux or custom firmwares and Google has to certify each firmware update from the manufacturer which takes several weeks each time. So basically a manufacturer has to give up all software control to Google with only a limited role left.
You can use apps like these for some casting ability for Youtube, Chrome browser etc. search online for the apk apps. Apps like Netflix will still block casting because of above.
https://play.google.com/store/apps/details?id=com.softmedia.receiver.castapp&h (Paid version helps)
https://play.google.com/store/apps/details?id=com.ionitech.airscreen
https://play.google.com/store/apps/details?id=com.apowersoft.mirror.tv
I made a LibreELEC image for RockPi 4 that also allows you to run Netflix up to 1080p quality.
Or if using Android, the Netflix addon for Kodi also works well if you setup Kodi’s whitelist and set up frame-rate switching so videos can play at the correct frame-rates, video playback will then be smoother and more natural compared to using the Netflix app.