Fwiw, Jidai Ishou no Nuikata has both bag-lined garments as well as
examples of garments lined as they were constructed (I don't. Know if that
is the "flat-lined" or something else). This appears to have been common
in the Nara period, when the garments were also more tailored.
There are also cases where the outside and inside are the same fabric,
folded over at the hem; especially some hakama and some hou.
Hems can be straight selvedge or rolled in unlined garments. Lined
garments might just meet, but more often, the outer garment is just a tad
bigger, so that the lining won't be seen.
I don't know that I have seen very complex stitching; usually just a
running stitch. They don't usually bother hiding thr stitching, but they
may at the hems, where more of a blind stitch might be used.
The thread, btw, is often undyed. Otherwise it will be a color to match
the fabric, if possible.
When joining two unlined pieces, it is often just a running stitch with the
sides splayed out and (sometimes) tacked down.
These are thoughts off the top of my head as I'm traveling and away from my
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