How to change default system shortcuts like Cmd Shift Q

I have to advise against changing these standard shortcuts, but if you want to try, just add a different shortcut - e.g. go to System Settings->Keyboard->Keyboard Shortcuts, click App Shortcuts, and add whatever you want to use - like Command-Option-Shift-Control-K for "Lock Screen":

Note: by deleting the custom shortcut, the default one is automatically reinstated.

Sleep, log out, and shut down shortcuts

You might need to press and hold some of these shortcuts for slightly longer than other shortcuts. This helps you to avoid using them unintentionally.

• Power button: Press to turn on your Mac or wake it from sleep. Press and hold for 1.5 seconds to put your Mac to sleep.* Continue holding to force your Mac to turn off.
• Option–Command–Power button* or Option–Command–Media Eject : Put your Mac to sleep.
• Control–Shift–Power button* or Control–Shift–Media Eject : Put your displays to sleep.
• Control–Power button* or Control–Media Eject : Display a dialog asking whether you want to restart, sleep, or shut down.
• Control–Command–Power button:* Force your Mac to restart, without prompting to save any open and unsaved documents.
• Control–Command–Media Eject : Quit all apps, then restart your Mac. If any open documents have unsaved changes, you will be asked whether you want to save them.
• Control–Option–Command–Power button* or Control–Option–Command–Media Eject : Quit all apps, then shut down your Mac. If any open documents have unsaved changes, you will be asked whether you want to save them.
• Control-Command-Q: Immediately lock your screen.
• Shift-Command-Q: Log out of your macOS user account. You will be asked to confirm. To log out immediately without confirming, press Option-Shift-Command-Q.