Which layer of the earth does the magma form?

There is no layer of the Earth that is made of Magma (molten rock). There is a layer called the Mohorovicic Discontinuity (or Moho) which is the boundary between the Earth's lower crust and upper mantle. The asthenosphere is a mechanically weak ductily-deforming region of the upper mantle which lies below the Moho. The asthenosphere is thought to be the source of some magma but it is not a complete layer of molten rock.