The construction of set shows that you are using the Make function with a module as argument. If you type Print Make., you see that this is a functor taking a module of type OrderedType as argument, and it produces a module with many fields, among which t, eq, eq_equiv, lt, lt_strorder, lt_compat, compare, compare_spec, and eq_dec. If you type Print OrderedType., you see that these field are the ones required to make an OrderedType.

So Make constructs all the fields that would be required to call Make again, thus producing the powerset.

You can just type :

Module pset := Make set.

and you will simply obtain a structure of sets, whose elements are in set.t. The following example was tested with coq 8.15.