It isn't. Exchanges don't set prices at all, they merely provide a place for people to trade with one another at whatever price they can agree on. If there are no customers, the order book is empty; if you like, you can think of the bid being zero and the ask being infinity. If the first customer is someone who wants to buy, they can enter a limit buy order at whatever price they want, and that will become the bid. When someone arrives who wants to sell, either their price matches the existing buy and a trade happens, or it doesn't and that price becomes the ask.
These are not really exchanges, just dealers. You are not trading with the company's other customers, but with the company directly. Just like an ordinary retail store: they have a product, they offer to sell it at a particular price, and their customers either buy at that price or take their business elsewhere.
And as general advice:
How to setup my own exchange?
If you have to ask the question, then don't. This is not a do-it-yourself project; the security and legal and financial considerations are daunting and need to be handled by experts. At best you will waste a lot of time before realizing that it is impractical. At worst you will discover it is impractical after you have lost a lot of money belong to yourself and your customers.