If you want to launch an app using a single key press, you can do it by creating an Automator Quick Action and assigning it a keyboard shortcut in System Preferences. This isn't usually needed for apps we always have running, but can be useful for apps we frequently bring up and then quit when not in use. You have to be careful not to use a name or shortcut that conflicts with any other. You also may find it is a bit flaky when you first create the shortcut.
I have always been reluctant to assign new keyboard shortcuts for fear of interfering with an existing keyboard shortcut. For instance, I want to assign the keyboard shortcut Command+W to launch a web browser. The issue here is that Command+W already exists in many apps and it is used to close a window. By assigning that keyboard shortcut, would the system alert me that the Command+W keyboard shortcut is already in use?
Ian: Yes. Try it and you'll see. You get a symbol next to it in System Preferences.
Gary: I tried it, I assigned Command+W for the new Launch Safari automator action (following your tutorial). I quit Safari and pressed Command+W and it launched Safari. I did notice that the close window shortcut changed from Command+W to Command+Shit+W. I removed that keyboard shortcut and Command+W now closes a window as it did before. But, I didn't see any type of warning or error on macOS Mojave 10.14.5.
Ian: I'm not seeing a warning either. But I'm sure there is supposed to be one. Note that Command+Shift+W is close window in Safari because Command+W is close tab.
Gary: Thank you for the confirmation, that lets me know that I didn't damage anything. Yes, Command+Shift+W is close tab, I read the wrong one. I do remember Linux and BSD having warnings for this type of thing.. might be a bug in macOS.