Why won’t my Apple password work on my iPad?

Your device passcode Is configured on each device. Whilst you can set the same passcode on multiple devices, the passcode is stored locally on the device - it does not synchronise between devices.

Your AppleID and associated password is shared across all devices. These credentials are stored on Apple’s servers.

Device Passcode is individual to each device.

Your AppleID and associated Password are common across all devices.

I really can’t be much clearer.

It is separate. It does not have to be different, but do you have to set it separately on each device.