@Dr Ms Cat Kat yes, knit fabric, I know of the knitting patterns for making underwear at home.

Or at least, I think that they are knit fabric, later today I'll look for the actual references, but I was thinking of knit underwear mentioned in mail order catalogues and books about healthy clothing.

@Dr Ms Cat Kat the things I was thinking of are like these: archive.org/details/cataloguef… which I'm quite sure would have been made in an industrial setting.

Now that I look at them I'm not so sure that they would have been made from flat fabric and not directly on a knitting machine, but I think some sewing would have been involved (the front plackets, and the sleeves also look sewn in the drawings).

Anyway, this wouldn't really be relevant to making these things at home, except possibly for the availability (or not) of flat knit fabric (which may not have been a thing, if these things were knit directly?)

@Dr Ms Cat Kat the internet also provided me with kristinholt.com/wp-content/upl… (ad for an union suit, based on patents from 1868 and 1878)

attributed as September 1878 Catalog of Novelties and Specialties in Ladies and Children’s Underwear on kristinholt.com/archives/5331 (which I didn't read, I only looked at the pictures and captions), which does mention “the vest and drawers are in one, being knitted together in the process of manufacture, forming a continuous garment from the neck to the wrists and ankles”, but also has a cheaper variant that is “cut and seamed”

@David de Groot I have used herringbone stitch on knits (to sew binding) and I can confirm that it works, and @Dr Ms Cat Kat reports that backstitch and overcasting work.

I don't think that sewing with a straight stitch sewing machine would work (or at least it wouldn't be durable), however (I don't know about the earlier chainstitch machines, but afaik they weren't that common late in the 1800s).