Free up space on Macbook Air 128GB

You can't completely.

How to free up storage space on your Mac - https://support.apple.com/HT206996

Optimized Storage [macOS Sierra or later] helps you save storage space space by storing your content in iCloud and making it available on demand:

- When storage space is needed, files, photos, movies, email attachments, and other files that you seldom use are stored in iCloud automatically.

- Each file stays right where you last saved it, and downloads when you open it.

- Files that you’ve used recently remain on your Mac, along with optimized versions of your photos.

iCloud is a conduit / means to have files in the iCloud and available to Sync from one device to another device provided all devices are signed into and using the same Apple ID.

iCloud was not intended to be like an extra External Drive and used for Storage.

Further, having only 1 GB of Empty Space - extremely dangerous. If left unattended to - the computer will stop loading at all. One needs to keep at least 15 GB of Empty Space Available at all times for the OS to load and the computer to perform normally.

What is “Other” storage on a Mac, and how can I clean it out?

How to free up storage space on your Mac

Buy an external Drive and start move Pictures, Videos, Music, Podcasts, old iPhone Backup off the drive to the external.

You've seen the link I provided (and was repeated in another post) on how to free up storage space on a Mac.

System files can take up a lot of space and it is tricky dealing with them. It is supposed to be self-managing but can often get to be very large and if you have a small drive such as you do it can take up disproportionate space. The category includes a lot of things best left alone such as caches. In that regard that's what the link to "other" storage is about, but in practice there's little you can do about "other":

See used and available storage space on your Mac [was: What is 'Other' and 'Purgeable' in About This Mac?] - https://support.apple.com/HT202867

"Other: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac."

On a notebook such as yours, System also includes Time Machine Snapshots. Ideally you should also have a Time Machine backup on an exterior drive and if you do, you can try turning off Time Machine temporarily on your interior drive and see if space is cleared up as Snapshots are removed.

Also check your iPhone/iPad/iPod backups if you use those.

Mojave or earlier : iTunes > Preferences > Devices > select backups you don't want, then Delete

I know Apple won't like me saying this because their take is that you should always be running the newest macOS. In your case I would review if this is wise. In particular, check the RAM on your computer. With a 128 GB drive I get the feeling you have an older Mac which may only have 4 GB RAM. While you can run Big Sur with that, Big Sur was designed with much newer computers with modern drive sizes and modern RAM capacities in mind. It's like theoretically I can carry a dishwasher in my car but do I want to really do that with my car, particularly on a daily basis? Anyway, just a consideration.

If you do not already have an external storage drive you may wish to consider getting one. Two in fact, one to back up the other.