How to see pianoroll from keyeditor in playlist?

I saw this in a cubasetutorial that you can edit key editornotes while being in the playlist at the same time. How do I do this?

By “Playlist”, do you mean the main Project window? There is no “Playlist” as such, unless you mean the Inspector of the Arranger Track.
Anyways, if you do mean the Project window, then what you probably saw was a MIDI or Instrument track with the In-Place Editor open (one of the icons in the tracklist on the left of the track… sorry, not at my music computrr right now, so can’t post a little screenshot). Anyways, when open, the Track expands to show something very similar to the Key Editor, but directly aligned with the rest of the project window.