How to use loops.foreach_set and polygons.foreach_set to add faces to a mesh

Question

I have problems getting my head around it:

I have two arrays:

verts = np.array([1, 1, 0,
                  2, 1, 0, 
                  3, 1, 0, 
                  1, 2, 0, 
                  2, 2, 0, 
                  3, 2, 0, 
                  1, 3, 0, 
                  2, 3, 0, 
                  3, 3, 0])

faces = np.array([0, 1, 4, 3,
                  1, 2, 5, 4, 
                  3, 4, 7, 6, 
                  4, 5, 8, 7])

The mesh (me) is prepared with the 9 verts from array "verts" in it already. Now i want to add the faces but the explanations i found were scarce at best.

So, how do i add faces using (an explanation would be great):

    me.loops.add() #should be 4 loops per (4-sided) face, right?
    me.loops.foreach_set("vertex_index",) #no idea...

    me.polygons.add(faces.size / 4) #4 Faces in this case
    me.polygons.foreach_set("loop_start",) #no idea... 
    me.polygons.foreach_set("loop_total",) #not sure... number of loops per face in a list: [4, 4, 4, 4] in this case?
    #me.polygons.foreach_set("vertices", ) #needed at all? List of vert coordinates if they were no verts in mesh already?

I would be very happy if anybody could shed some light on it for me.

EDIT After some investigation of "from_pydata" code in "bpy_types.py" as suggested by batFINGER i am a bit further but still not fully able to create the correct mesh.
I now have most of it right i think but because i have shared vertices for some of the loops and edges i am now pretty sure I need to use:

me.loops.foreach_set("vertex_index", vert_idx)

i thought i would need to add two verts per loop (start and end), so:

vert_idx = (0, 1, 
            1, 4, 
            4, 3, 
            3, 0, 
            1, 2, 
            2, 5, 
            5, 4, 
            4, 1, 
            3, 4, 
            4, 7, 
            7, 6, 
            6, 3, 
            4, 5, 
            5, 8, 
            8, 7, 
            7, 4)

or "faces" but both leads to a crash, like everything else i tried.

Answer 1

Got it, what was missing was not:

me.loops.foreach_set("vertex_index", vert_idx)

but correct:

me.polygons.foreach_set("vertices", ...

i tried it before and it crashed all the time because i forgot to ravel() the numpy array i used.