This one[1] shows how the teeth are sewn on, which does look like it would be hard to do by hand (and impossible with a normal sewing machine).

1: https://ykkdigitalshowroom.com/assets/80006b690a9e47db1e62ad...

OK, finally I see why a new sewing machine was needed. Thank you! That's a sort of quasi-saddle-stitch, and one mistake in timing the length and 'phase' of the stitch means a broken needle.

Another view: https://imgur.com/a/FQGdSkg