There is no way to disable the built-in keyboard that I'm aware of except perhaps through the use of a third party app. See this article for Karabiner-Elements app where you can disable the built-in keyboard when an external keyboard is connected. Keep in mind that you won't be able to have the external keyboard sit on the built-in keyboard while the laptop is booting since this app won't be loaded until later in the boot process. I've seen a lot of people recommend this app for customizing keyboard behavior with macOS, but I have never personally used this software myself so use at your own risk.
https://karabiner-elements.pqrs.org/docs/manual/configuration/disable-built-in-keyboard/
Be very careful placing anything above the laptop's built-in keyboard since you don't want to accidentally close the display when something is in the way since it will likely result in the Display becoming damaged. I've seen so many of our organization's laptops where someone accidentally closed the Display when something was in the way. It is an extremely expensive repair if the Display must also be replaced.