To quit (close) a Mac app normally, choose Quit from the app's menu in the menu bar, or press Command (⌘)-Q. If the app doesn't quit, follow these steps to force the app to quit.
Using Homebrew To Run Unverified Apps on Mac While Apple would prefer you to install apps through the App Store, you can bypass it completely with Homebrew. The benefit of using Homebrew to install macOS apps is that it bypasses the security mechanisms that Apple uses to “protect” you from unverified apps. Show All Running Apps On Mac Using Force Quit Applications Manager. Another method to check all the Running apps and programs on your Mac is through the Force Quit applications manager on Mac. Click on the Apple icon in the top menu bar of your Mac and then click on Force Quit Application in the drop-down menu (See image below). CodeWeavers’ CrossOver Mac is a paid application that will run Windows programs on Mac. It uses the open-source Wine code to accomplish this, but CrossOver provides a nice graphical interface and focuses on officially supporting popular programs. The 32-bit app appears in its own window on your Mac desktop, and a second top-line menu (the menu for the virtual Mac) appears beneath the top-line menu of your main macOS installation. As you can see in this window, my Mac is running Catalina, but a 32-bit app is running in a window that looks like any other app window.
Show All Running Apps On Mac Using Force Quit Applications Manager. Another method to check all the Running apps and programs on your Mac is through the Force Quit applications manager on Mac. Click on the Apple icon in the top menu bar of your Mac and then click on Force Quit Application in the drop-down menu (See image below). If I understand correctly, you want to install apps but not the OS on the external drives. You can do this easily but not for apps downloaded from the App Store.
How to force an app to quit
Press these three keys together: Option, Command, and Esc (Escape). Or choose Force Quit from the Apple menu in the upper-left corner of your screen. (This is similar to pressing Control-Alt-Delete on a PC.)
Then select the app in the Force Quit window and click Force Quit.
Mac Os App Runner App
You can even force the Finder to quit, if it stops responding: Select Finder in the Force Quit window, then click Relaunch.
Learn more
Mac Os App Runner Tutorial
How to force an app to close on your iPhone, iPad, or iPod touch.