New explicit construction of fold maps on general 7-dimensional closed and simply-connected spin manifolds

7-dimensional closed and simply-connected manifolds have been attractive as central and explicit objects in algebraic topology and differential topology of higher dimensional closed and simply-connected manifolds, which were studied actively especially in the 1950s--60s. Attractive studies of the class of these $7$-dimensional manifolds were started by the discovery of so-called exotic spheres by Milnor. It has influenced on the understanding of higher dimensional closed and simply-connected manifolds via algebraic and abstract objects. Recently this class is studied via more concrete notions from algebraic topology such as concrete bordism theory by Crowley, Kreck, and so on. As a new kind of fundamental and important studies, the author has been challenging understanding the class in constructive ways via construction of fold maps, which are higher dimensional versions of Morse functions. The present paper presents a new general method to construct ones on spin manifolds of the class.