How can I add custom items to the Apple menu? I would like to add shortcuts to open apps such as Terminal, Google Chrome, Launchpad etc., or to open a folder as a sub-menu.

Is it possible to do so? If yes, how do I do it?

I am running macOS Mojave.
The Apple menu is not designed to be customized by anyone. However, I would recommend XMenu (available on the Mac App Store, and also download from here).

This handy little app will give you menus in the menu bar that you can customize, either to include the entire Applications folder as a drop-down menu, or you can add aliases to 'Favorite' items.