Pressing "i" puts the cursor in the "Add Note" input in the YouTube widget.
Pressing "o" collapses the YouTube widget. Pressing "o" again will expand the widget.
Pressing "p" adds a pinned note with no text.
Pressing "[" toggles Edit mode. This will allow you to click the edit or delete buttons next to a note.
Pressing "SHIFT + 1" or "SHIFT + 2" on a YouTube playlist or live video page changes which tab is active in the YouTube widget.
Pressing "z" opens this menu, where you can view keyboard shortcuts and docs.