"The public" means "most people". If a store is "open to the public", it means that anyone can come into the store. If a store isn't open to the public, it's either closed or restricted to private use.

If something is "available", it means that you can get it or use it. So "available to the public" means that anyone is able to buy it. Use this to talk about products that are being sold to a large number of people.