is there a good app for finding wall studs?

iPhone and iPad do not contain sensors necessary for detecting density or metal.

Rapping on a sheetrock wall with your knuckles and listening to the sound usually works well enough.

Align the hollow and not-hollow sounds in the wall and looking or feeling for the fasteners, and off you go.

Or buy a stud finder.

Stud fingers also commonly have voltage sensors, which can be handy when performing certain repairs.