Storage full warning on my MacBook Air

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From Free up storage space on your Mac - Apple Support,

Find out how much storage space is available

Your Mac can show you how much storage space is being used by various categories of files, and how much space is available for additional files. As you take the steps in this article to free up storage space, this storage information updates automatically. 

macOS Ventura or later: See used and available storage space

Earlier versions of macOS: See used and available storage space

Optimize storage space

Your Mac can optimize storage by using iCloud to automatically make more storage space available when needed.*

Let your Mac free up storage space for you, and use its built-in utilities to quickly find files, apps, books, movies, and other items that are taking up space, then delete items you no longer need.

macOS Ventura or later: Learn how to optimize storage space

Earlier versions of macOS: Learn how to optimize storage space

Move or delete files

If you have another storage device, such as an external drive connected to your Mac, you can move files to that device. Media files such as photos and videos can use a lot of storage space, so you can take steps such as these:

Move your Photos library

Move your iMovie library

You can also delete files that you no longer need. For example:

Delete music, movies, podcasts, or other media, especially if it's media that you can stream or download again as needed.

Delete files in your Downloads folder. Open this folder from the Dock, or by choosing Go > Downloads from the menu bar in the Finder.

If you use the Mail app for your email, choose Mailbox > Erase Junk Mail from the menu bar in Mail. Also choose Mailbox > Erase Deleted Items.

If you use your Mac to back up your iPhone or iPad, you can delete old backups.

Uninstall apps that you no longer use.

If your Mac is set up for multiple users, delete users that no longer need access to your Mac. Or ask them to follow the steps in this article after logging in to their user account. Also check the Users folder on Macintosh HD for any undeleted data belonging to deleted users. 

Check the Users folder on Macintosh HD for any shared files that you no longer need to share.

If you don't want to delete certain files, you might be able to save space by compressing them. Then expand them when you need to use them again.

Jack