For the power button, go to Settings > Advanced features > Motions and gestures, then turn on the following settings:
Lift to wake
Double tap to turn on screen
Double tap to turn off screen
For the volume buttons, just download One Hand Operation+. It controls volume buttons and other buttons. Did the Bixby button fall off?
I'm sorry this happened to you. I suggest you bring your device to an Authorized Samsung Service Center. Our Samsung techs are well equipped to help you with your issues. Here is the Samsung Service Locator so you would know the nearest service center in your area: https://www.samsung.com/ph/support/service-center/?msclkid=72d47e20cf8411ec8ac1394658c823b0