Official Entry: "Manufacturing is largely automated but the ideas for new products still come from human designers. Synthemesh employs many designers to keep generating the latest fashions which fuel consumer demand. The manufacturing plants produce mostly consumer durables, but the corporation also controls a fleet of construction vehicles which are used to build or repair the city infrastructure."
Synthemesh does not sell any products useful to X-COM.
Synthemesh does not have any predictable starting pattern of relationships with other organizations. Other than being the ones responsible for rebuilding the damage inflicted by fighting they play no other role in the game unless provoked.