Creating cascading menu commands

You can edit those menus via Settings > File Types.

For moving/removing items added by other software, see: Tip: Organise and Speed-Up Context Menus

But the right-click menu for drives/folders/files are not where things which like "Select All" which work on the current folder really belong. Those menus should contain items which work on the thing which was right-clicked.

The background menu for the folder is the proper place for things like "Select All", and that can be edited via Settings > Customize Toolbars > Context Menus.