Sadly I can't really understand how they are stitched.
If we look at the interior, there's no visible stitching for the zipper so I thought the bottom was added in the end.
But is it possible to finish sewing the pannels with such a bottom plate ?
I'm really really curious to hear how would you construct such a bag ?
Do the vertical strip on the sides play a role ?
What are the first pieces to be sewn together ?
If I had to do it, I think that I would stitch the sides to the bottom first. Then the main zip to those sides. That done, I'd stitch those strips shown on the narrow sides in order to finish forming the bag (though it might be difficult to do so, particularly at the bottom) and I'd finish sewing the drop-in lining. A lined plate seem placed at the bottom on the inside that could help masking the final stitching of the lining.
Thanks a lot for your ideas !!