Apps · The Apps window and launcher
Is there a keyboard shortcut to open the Apps grid from anywhere?
Short answer: Yes — press ⌃Space (Control-Space) by default to open the All Apps grid from anywhere. You can change it in Settings → Advanced → Keyboard Shortcut.
Yes. A global keyboard shortcut summons the All Apps grid from whatever app you're in. It's ⌃Space (Control-Space) unless you change it.
You can change it in Settings → Advanced → Keyboard Shortcut, or by clicking the small shortcut keycap next to the heading on the Home screen. See how to change the shortcut.