my nav_vertices and nav_polys are getting created on runtime while i procedurally create my worldmap. if you do someting like that, then you already should have them. you dont get them, you create them.
if your map is created in the editor you dont need to create the navmesh on runtime. see the godot example application for navmesh. you only need the code stuff if you have an random map like i do!
about nav_vetices and nav_polygones
well nav_vetices is an Vector3Array.
imagine a pathway out of polygones, nav_vetices contains the position of your points.
but nav_vetices has are only the points! they dont contain the information how to connect them. for example a quad:
v1 = Vector3(...)
v2 = Vector3(...)
v3 = Vector3(...)
v4 = Vector3(...)
now the quad could be build as a quad itself or 2 trinangles. with only v1-v4 godot cant know what you want!
in this example i choose 2 triangles, so i need and indice list to tell godot how v1-v4 are connected:
the index is the the same index as the order you add the vector3 to the Vector3Array. So 1=v1,2=v2....
nav_polygones in my example is the list of lists of indices like:
i hope my explaination is sufficent. and sorry for the poor english.