I know you can turn a MacBook's brightness essentially off, just by bringing it to the lowest tick. On iOS there is a VoiceOver gesture to turn off the screen even while the phone is on, so there may also be a purpose-built version of that in VO for macOS.

edit: Evidently there is VO-specific tooling: https://support.apple.com/en-us/HT201443

Most PC laptops have a key in the function key or media control row for this. Look for one that resembles a monitor, or check your laptop's owners' manual.