Catalina Quick Tip
macOS Mojave added a new feature to the Dock: it now shows your three most recent apps. This section sits between your collections of app icons on the left and the section of the right that shows your open apps.
I find it convenient; however, you may not. If so, you can disable this feature. Go to System Preferences > Dock and uncheck the box at the bottom for Show recent applications in Dock.