To power off the iPad, you would simultaneously press and hold the top button and either volume button, then drag the slider.
If this wont work, then the device is unresponsive and needs to be forced.
To force a power off by pressing the top volume button, then press the bottom volume button, then hold the power button.
Resouces:
https://support.apple.com/guide/ipad/turn-ipad-on-or-off-ipad63d30b5a/ipados
https://support.apple.com/en-us/102642