Giant makes a lot of frames for other companies. They make at least some of Trek's aluminum frames. It's pretty obvious they make their kid's bikes.
Pinarello makes Eddie Merckx frames.
The Kent/Pacific/etc. company makes all the x-mart bikes.
This is quite common for computers; the company (eg. Apple, Dell) requests bids from various contract manufacturers to meet specifications for their products.
Also quite common for food companies. They buy from a cannery and have the cans labeled with their name on it.
My brother-in-law did some welding work for a large brewery in Mississippi. They had twenty-two different brands there but only made 11-12 different beers. Everything from Miller Lite to Sapporo. He didn't know which beers were actually the same.
Honda had their Passport SUV made by Isuzu. Ford made Mazda pickups. Toyota and Chevrolet share a plant in California; churns out Novas and Corollas, only difference being the name badge.
RRich, why do you care?