Previous I thought that this is something only Apple apps can do, but I just noticed that Google drive can do that too. But I couldnt find the documentation for that. Does anyone know? thanks!
How to display the “Search in App” button in Spotlight?
You need to include the CoreSpotlightContinuation key in your Info.plist - it's a boolean that you should set to YES.
https://developer.apple.com/documentation/corespotlight/csquerycontinuationactiontype