startup disc is full macbook Air

Try OmniDiskSweeper. It will tell you the files and folders which are using the most disk space.

What ever you make sure that you have at least one backup before you start cleaning the disk drive. Two backups would be better.

Hello,

Disk space can be managed by pulling down the Apple Menu  -> About this Mac -> Storage.

Hello,

This is used by the Macintosh system software and any applications or data that was not downloaded from the App Store.

Yes.