There are two different magnetic base hooks. Depending on the type of substrate, they can be applied directly magnetically.
Attaching magnetic hooks to a metal surface will seem obvious to you, but an indirect way is now also possible.
Via a so-called GoodHangUps Magna sticker, a metal plate can be applied to a flat wall (including concrete or stone) without nails or screws. This immediately lays the foundation for the magnet that clamps photos or images between the applied sticker.