It’s supported with money. Lots of money.

The design plan for something like that starts in the framing stage, and uses stainless steel supports in the framing.

It depends on the material and the width of the ledge. Engineered stone is much stronger than any natural material and will cantilever safely with less support.

I've seen this at one of my sister's houses in Europe. Her stone shelf is supported by a thick stainless steel shelf (in an upside down L shape) that reaches down behind the backsplash. The stone shelf itself is either routed out to make room for the steel to be hidden, or it might have a mitered front edge, I'm not sure. The steel is not visible unless one crouches down and peers up. Her shelf is rock solid and could support much more than the couple of cups, glasses, small plates and framed artwork that she keeps on it. By the way, it's hanging on a concrete wall, no studs and spans the length of the wall.

so this upside down L is continuous? As well on a non concrete wall it would be drilled and mounted onto the studs-- And then the backsplash material goes up to hide all of this?