My MacBook air did not come with Boot Camp assistant. How can I download a copy of Boot Camp assistant

Every MacOS includes Boot Camp Assistant. Boot Camp can not be used on M1 Chip Macs.

Boot Camp - Official Apple Support

If it is a new M1 chip Mac, that does not support Boot Camp.

Apple menu -> About This Mac will tell you if it is Intel or Apple M1 processor.

https://discussions.apple.com/docs/DOC-2741

lists several other programs which are working on running Windows on Apple's new processor.

Boot camp was not on 10.5.8 or earlier PowerPC either. Not that it matters here. But "Every" Mac OS should as much include the the older Mac OS versions that didn't as the newer ones.