Foxconn and Hon Hai are manufacturers (like the summary says). Apple (and other tech companies) work with them the same way you'd work with, say, a metal foundry somewhere to have them manufacture some parts you've designed. The main reason for this is you can't just invent something (especially technology), and just ask people to build it for you, they need to be able to.
So, this wrist-contoured glass. It sounds like Foxconn has been working on a new process for this, and have been looking for a way to get customers to pay them to build stuff for them using it. In this case, working with Apple to work it into a new watch.
The engineering for the watch itself will be all Apple. The engineering behind how to manufacture it will be a collaboration between Apple and Foxconn (or whoever), the same way Apple (and everyone else) has always done it. This is similar to how the processes for manufacturing the unibody aluminum enclosures and the iPhone 5 parts matching system was developed.
What you're thinking is a private label system, with some companies engage in (not Apple), where you might see the exact same product that gets labeled with different brands from different companies, similar to how some store brands (especially food and household supplies) are done.