Mac actually running out of space due to Other

Ever since Apple changed the filing system and decided to “sequester” the OS and all system files on its own inaccessible read only volume, controlling this category has been taken over by the OS:

See used and available storage space on your Mac – Apple Support (UK)

FWIW, it has now been renamed system data in Monterey, but you still cannot control it.

See used and available storage space on your Mac – Apple Support (UK)

Free up storage space on your Mac - Apple Support

If you need to make more storage space available on your Mac, use its built-in tools to optimize storage space, or free up space in other ways.

1. From the Apple menu  in the corner of your screen, choose About This Mac.
2. Click Storage. At first the storage bar estimates the amount of used and available storage space. When it finishes calculating, it shows segments representing several categories of files.

3. Click the Manage button to open the storage management window:

4. To begin optimizing storage space, choose from features such as Store in iCloud,* Optimize Storage, and Reduce Clutter. Learn more about these features.

5. To measure progress as you make changes, refer to the amount of “available” space shown at the top of this window, or in About This Mac.

Free up storage space in other ways

• Delete unneeded media, such as music or movies or podcasts, which can use a lot of storage space.

• Delete unneeded files in the Downloads folder. You can open the Downloads folder from the Dock or by choosing Go > Downloads from the menu bar in the Finder.

• Delete unneeded email in the Mail app. To delete junk mail, open Mail and choose Mailbox > Erase Junk Mail. To delete mail that you've moved to the Trash, choose Mailbox > Erase Deleted Items.

• If your Mac is set up for multiple users, remove users that you no longer want to have 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 previously removed users.

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

• Move some of your files to external storage, such as iCloud Drive* or an external drive connected to your Mac.

• Uninstall apps that you no longer use. 

• Compress files that you use infrequently.

Remember that when you move a file to the Trash, its storage space doesn't become available until you empty the Trash.

* Storing files in iCloud uses storage space in your iCloud storage plan. If you reach your iCloud storage limit, you can Upgrade to iCloud+, or you can Manage your iCloud storage. iCloud storage starts at 50GB for $0.99 (USD) a month. you can purchase additional storage directly from your Apple device. Learn more about prices in your region

I have found these two things to be among the usual culprits in "Other." Both are stored in locations inside ~/Library.

• iOS device backups (they are full copies of your iOS device(s))
• Email if you use Outlook, Thunderbird or other non-Apple email client. (Mail also stores email in ~/Library but Manage Storage shows it on its own.)

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. 

I've don't use OmniDiskSweeper. I prefer the graphical representation of Grand Perspective which shows all files on the hard drive.

You don't need to worry about the "apps in use". You need to determine which files you can delete permanently or move to an external drive for long term storage to free up additional space.

One of the reasons I decided not to upgrade at this time (until I am forced to).