In order to show or hide the Navigator, Debug area, and Utilities, use the following shortcuts.
- Navigator: Command+0
- Debug area: Shift+Command+Y
- Utilities: CommandOption+0
(Those are zeros, not the letter O.)
Here is an image from this article (which itself was adapted from this blog post):

Useful links about Xcode shortcuts:
- Xcode Efficiency Tips: Keyboard Shortcuts
- 14 Xcode Time Saving Keyboard Shortcuts – Memorize and Improve Your Productivity
- 11 Xcode Keyboard Shortcuts to Make You More Productive
- Menu Command Shortcuts