i just bought a macbook Air how do I know what system is on it

On the Desktop Apple Icon >> About this Mac >> next windows opens and will tell what version of macOS is installed. ( ie: Big Sur, Catalina, Mojave ec. )

If it's new and bought from Apple then you have macOS Big Sur.

If it's second hand then click the menu > About > here

Hi,

To see your the operating system of your MacBook, follow these instructions.

Hi,

This article will help you.