need some help with generating geometry matrix

hi!
i am trying to create simple cube by sending geometry matrix to jit.gl.mesh. i have read a lot about geometry matrices, verticles and stuff like that but still cant find the clue. i rid that when drawing objects using draw_mode tri_grid the verticles are connected either to neighbor from the same plane and the other plane.
i thought that i will create rectangle of verticles with constant z value 1 in the first dim, and then just copy values, changing z to -1 into the second dimmension. the result is something like unfinished, open cube.
so my questions are:

-why would we even need 2 dimmension matrices for geometry?
-why the smallest dimmensions to draw the cube correctly is "dim 8 8"? isnt 64 points for creating simple shape quite lot?
-in my patch under "matrix data from jit.gl.gridshape" comment there are jit.cellblock. why a lot of values are repeated?
- what for there are two verticles in the middle of two sides?

thanks for advice!
adam

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