Start-up Disk gone from MacBook Air

Hi,

Boot in Recovery Mode then reformat internal drive and reinstall macOS.

Use macOS Recovery on an Intel-based Mac - Apple Support

Use macOS Recovery on a Mac with Apple silicon - Apple Support

OWC sells a drop in replacement SSD which is compatible with the proprietary Apple SSD connector. However, to use a third party SSD requires the laptop previously had its system firmware updated to work with an NVMe SSD which means the laptop at one time had macOS 10.13+ installed on it. Plus you will need to use macOS 10.13+ with the new NVMe SSD. If the laptop is compatible with macOS 12.x Monterey, then you will only be able to install Monterey if macOS Monterey had already been installed on the Apple SSD since the Monterey installer won't update the laptop's system firmware the first time it is installed without an internal Apple OEM SSD.

https://eshop.macsales.com/shop/ssd/owc