Were there clothing designers who set the trends for what was popular to wear in that time period? Who are they, and how would they accomplish it?
I mean, somebody had to design all the elaborate clothing of the 17th century, do we know who they are?
For a long time there wasn't much distinction between designers and tailors/dressmakers. Clients would tell the tailor what they want and the tailors would make suggestions or alterations. It is generally agreed that Rose Bertin was the first famous designer with her own distinct profile and aesthetics and made France the capitol of fashion. She befriended Marie Antoinette and soon became her official dressmaker and earned her endorsement (thus creating her own "label" and reputation) and other wealthy clients from France and even abroad would came to her for new dresses. She would also outfit dolls with her clothes and send them around the world for commission or inspiration. She was called "Minister of Fashion". (You can read more about her from her biography).
The father of Haute Couture was English designer Charles Worth. He was the considered the first fashion designer in the modern sense. He employed many tailors and seamstresses to turn his designs into clothes (most tailors back then were commissioned by patrons directly). He was also one of the first clothing maker to put his own label on clothes to establish his reputation.