Just about all macOS users are aware that you can add items to the dock by dragging the item from Finder into the dock. There is a quicker way by using a handy keyboard shortcut.
First select the item in Finder, be it an app in the Applications folder, a file or document, or even a folder. Enter Command+Control+Shift+T. The selected item will now be in the dock. Applications will be added to the left/top side of the dock, whereas documents or folder will be at the right/bottom end of the dock.