What even is upholstery? 
It is the covers of furniture.  This includes the padding, springs and fabric
Ahh okay, that's kinda what I thought it was.
The word actually comes from the Middle English.  It means upholder
That makes a lot of sense as to the root of it.
Upholstry has changed a bit in modern times.  It often uses sythetic material like vinyl and acron
That outta be a lot cheaper and easier to produce.
An apprentice upholsterer is sometimes called an "outsider" or "trimmer". Traditional upholstery uses materials like coil springs (post-1850), animal hair (horse, hog and cow), coir, straw and hay, hessians, linen scrims, wadding, etc., and is done by hand, building each layer up. In contrast, today's upholsterers employ synthetic materials like dacron and vinyl, serpentine springs, and so on.
History.
"Upholder" is an archaic term used for "upholsterer"
Upholstery
Upholstery is the work of providing furniture, especially seats, with padding, springs, webbing, and fabric or leather covers. The word "upholstery" comes from the Middle English word "upholder", which referred to an artisan who held up their goods. The term is equally applicable to domestic, automobile, airplane and boat furniture, and can be applied to mattresses, particularly the upper layers, though these often differ significantly in design. A person who works with upholstery is called an "upholsterer".