How to implement calling the application on pressing hotkeys and passing arguments to it from the Finder?

Question

macOS

I'm just getting started writing Mac apps. I went through several lessons, I smoke the documentation on developers.apple.com, but so far I have no idea how to correctly implement one simple (?) idea.
By pressing the hotkeys, my application will be launched, which simply has a text field containing the names of the files that are selected in the Finder (if any). With these files, I then want to do different things.
Naturally, I'm not asking about a specific implementation, but it would be useful to know which concepts to read more about and, perhaps, which classes to pay attention to.
macOS 10.13.4, Xcode 9.3.1, Swift 4