I just don't understand how the code doesn't fail when the map looks to be created after the code before is executed...

The confusion seems to come from misreading the body of the arrow function:

foo((map, v) => map.set('foo', v), new Map());

In the code above, map.set('foo', v) is not called before new Map() because it's inside the anonymous function passed to foo.

Maybe rewriting like the following will help you understand:

const f = (map, v) => map.set('foo', v);
foo(f, new Map());

check down three posts!

More about string interpolation on MDN. The only purpose here was to convert number to string (which happens implicitly within backticks), and this is just one of many ways to do it.

this is JS version of string interpolation (a bit weird to use it there, tho)