Navigate to the handbook page for Musescore 3: Barlines
Change bar line type
Bar lines are changed by dragging a bar line symbol from the bar line palette to a bar line in the score.
To hide a bar line entirely, select the line and untick Visible in the Inspector (F8).
Create grand staff (great stave)
To extend bar lines over multiple staves, double-click on a bar line (see Edit mode).
Click and drag the blue handle down to the next staff.
The staff bar line updates appear after leaving edit mode.