No one here knows, or is going to tell you how exactly they do it.

I could make an educated guess though;

1. they sell BTC for their giftcards.
2. they then resell the giftcards, or services that the giftcard provide to a third party with profit.
3. Use profit/returns to buy giftcards again.
...
4. Profit.

Now how exactly vendors create these traderoutes, is something they're probably not going to tell you, otherwise they'll soon be out of business/get less profit. It's really up to you to find something that works.