image/svg+xml
I will create buttons for +/-, to be used to increase/decrease values for example.
First create a rectangle with round corners
Now duplicate the rectangle and rotate it 90 degrees, and join both using Path->union.
Add stroke to the path using the button inner border color.
Duplicate the path and change to black color with transparence to make the shadow.
Repeat the steps to make the minus/decrease button.
Volume:
First create a rectangle with round corners
Duplicate the rectangle and change its color to a darker color
Add darker stroke to the inner rectangle
Duplicate the inner rectangle, remove the stroke and make it transparent.
Cut the new transparent rectangle at the middle using another rectangle and path difference.
Modify the transparent rectangle shape to make it smoother and adjust the borders.