How to reboot a frozen Android device? When an Android device locks up, and does not respond to holding down the power button, is there anything you can do to reboot the device besides removing the battery?

What do you do if the battery is not removable?

Although it does not always work, Windows has the three finger salute (Ctrl+Alt+Delete) which helps get around this situation.  Is there anything similar in Android?

I did find this question, but it does not refer to a device that is frozen.
Different Android devices may have different force-reboot methods, so you should refer to your device’s documentation. To reboot frozen device you should usually press and hold the Power button + Volume Down button (or Volume Up button - it depends on device) for at least 7~10 seconds and stop holding those buttons once it’s started the reboot process. You could try to press and hold just the Power button itself for at least 30~45 seconds.