I may purchase a 2015 MacBook Air.Can I upgrade a MacBook Air? ( OS, Storage, etc…)

The model year of a MacBook Air is available at Apple menu > About This Mac.

An early 2015 or later can be upgraded to the current version of Monterey macOS 12. See: How to upgrade to macOS Monterey - Apple Support

Memory cannot be upgraded but storage is upgradeable. One well-respected source is https://eshop.macsales.com/shop/ssd/owc/macbook-air/2013-2014-2015

The RAM you get is the RAM you will need to live with.

The SSD can be upgraded, however, it is not a standard interface. There are however a very small handful of vendors that carry upgrades (OWC for one and usually from many here the most trusted).