How to free up storage space on my Mac?

See used and available storage space on your Mac

Especially the sections regarding " Other " usage and the final word from Apple of that topic.

• 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.

Some very serious house cleaning will need to be done and / or moving Documents, Photo, Podcast, Music, Videos, old iPhone backups

I had a similar issue that was caused by a giant log file of email server connection data in Library. To find it, I had to show hidden files in Finder. Then I used the File Browser tab in Manage Storage to find the offending file.

Consider the options presented by @ sberman in an earlier posting. It may seem difficult but can be done. Just be certain to have a Rescue Plan in-place and current.

A Rescue Plan and has kept my computers in good order for many years. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the wipe and Reinstall process goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

Is this the same as the "Other" you get with ➙ About This Mac ➙ Storage? If so then the following may help:

The Other category is a potpourri of files which can include:

• System temporary files

• macOS system folders

• Archives and disk images (.zip, .iso, etc. - often found in the Downloads folder)

• Personal user data

• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)

• Cache files: browser, Mail

• Mail messages & attachments

• Fonts, plugins, extensions

• Safari reading list

• iTunes backups

• Crud resulting from jailbreaking your iDevice

• Game data

• Saved data files

• Call history

• Notes

• Media

• Voice memos

• Other files that are not recognized by a Spotlight search

• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package

• Files created and modified by other user accounts on your Mac.

They can be located anywhere on your hard drive.

The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use either of these two free apps, GrandPerspective  or OmniDiscSweeper, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.  

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Many users have found a couple of Gigabytes of files in their Downloads folder which are no longer needed. 

How to free up storage space on your Mac - Apple Support

You could erase and reinstall MacOS.

Erase and reinstall macOS - Apple Support