Q: Macbook Air pcle-based flash storage

That is the internal storage. Instead of what used to be typical in computers -- a spinning hard disk drive -- the MacBook Airs come standard with flash storage. I view it as a substantial improvement as they are very fast. They are becoming more common even on Windows computers but usually as an option at an added cost.

You can attach an external drive if desired -- via USB the same as other computers.

Something like following one is the main internal storage of MacBook Air.

https://eshop.macsales.com/shop/ssd/owc/macbook-air/2013-2014-2015