SmartTube is not officially available on Chromecast with Google TV from the Play Store
If your Chromecast only shows photos, it may be in Ambient Mode or not fully set up
Set up the Chromecast with the Google Home app first
Make sure the Chromecast is connected to Wi-Fi and linked to your Google account
On the Chromecast, open Settings
Go to System
Go to About
Check for system updates and install them
On your phone or computer, download the SmartTube APK from the official SmartTube GitHub release page
Copy the APK to a USB drive if your Chromecast supports USB OTG
Or use a file manager and wireless transfer method if USB is not available
Install a file manager app on the Chromecast if needed
Enable Developer options on the Chromecast
Turn on Install unknown apps for the file manager or downloader app
Open the APK file on the Chromecast and install SmartTube
If installation is blocked, use ADB sideload from a computer
Connect the Chromecast to the computer over ADB
Run the install command for the SmartTube APK
Open SmartTube from the Apps section after installation
If the Chromecast only shows photos and no app interface, restart the device after setup
If SmartTube still does not appear, verify that you have Chromecast with Google TV and not a standard Chromecast model
Standard Chromecast models cannot install apps directly
Use a TV or monitor with Chromecast with Google TV for app installation and use