Right. Once you are editing a Part you are editing all the data in that Part.
However you can set the notes so the color is determined by Channel instead of the default Velocity, which makes it easier to tell what’s what.
If each each Channel was in its own unique Part then you could select and edit all the Parts at once but restrict the editor to only allow changes to the active Part. By changing which Part is active you can select what channels you are editing.