But if I target 3:2 aspect ratio, my game should work an almost all devices, but a user will see black bars around my game if device aspect ratio is different. Am I right?
First, did you look at the links i posted before?
Basically, if your layout is....say 600x300, and your window is....say 400x200, you want your layout, that 600x300 to be a bit larger. Say 700x400.
Then when you add objects, make sure NOTHING important is close to the edges.
Again, i can not remember the exact expression you need here.
Its either viewport or window expressions. And there will be A LOT of trial and error here.
I do not know if you could actually bounce the ball off the edges of the screen. As i said, adding plain objects to your game then making them transparent and rectangle and placing them at the edges of the screen by way of the viewport or window expression.
(This is wrong, only adding it for an example)
create object - X position.viewport.left +30
Again, that is not the expression but its something like that.
Search the manual for viewport and windowwidth and or do a forum search. Take some time to read through.