What version of MacOS ships with new Mac Air

That's not such a simple question as it sounds.

The model isn't quite as "new" as it was. The current MacBook Air was released in November 2020 concurrent with the release of MacOS 11 Big Sur. These computers were sold with that operating system.

The subsequent system MacOS 12 Monterey was released on 25 October 2021. Any Macs in the supply chain between the factory and a retailer retained the OS which was current when it was in the factory - Big Sur. By now, five weeks later, the last of the stock with Big Sur should have been sold, but there may be some with Big Sur.

As an example, mine was delivered to the UK from China on the 4th November. It came with Monterey 12. By that time, we were on Monterey 12.0.1 so the update was the first thing I did.