How can I find out what specific items contribute to my MacBook Air storage described as 'Other Users & Shared'?

Shared items are in the Shared folder, within Users: Folders that come with your Mac - Apple Support

"The Users folder also contains the Shared folder."

The contents of other User Accounts on that Mac are also in Users. If you are the sole user of that Mac then those 34 GB are yours.

iPhone contents are not a factor except for iPhone backups: Locate backups of your iPhone, iPad, and iPod touch - Apple Support

6 GB of available space is getting a bit low: Optimize storage space on your Mac - Apple Support

If it's not immediately obvious by inspecting the contents of that Mac's Users folder (see Folders that come with your Mac) you can use an app such as Daisy Disk or Grand Perspective, either of which can provide a graphical representation of large files to consider deleting. They are in the App Store.

Caveat: I do not use them. Find and delete files on your Mac provides sufficient detail. Those two apps are just suggestions for your consideration. Never use any app or utility other than what macOS provides (specifically, the Finder) to delete files.

Needless to say, don't do anything without a backup... but 6 GB of free space might not be sufficient for Time Machine to store the local snapshots required for it to work.

Back up your files with Time Machine on Mac - Apple Support

If the need arises offload large files to external storage first: Connect and use other storage devices with Mac - Apple Support. Then, macOS may require a day or so for it to reconcile and release free space and make it available. Hopefully that's getting ahead of ourselves.

It’s awesome that you’re curious about what’s taking up so much space on your drive!

And it’s fantastic that @John Galt and @ BobTheFisherman have offered their help and tried to explain what might be in the System Data (Other) category.

However, if you take a peek at some of the other posts like the ones below, you’ll see that reducing System/Volume/Data is a pretty common question.

Here’s a helpful resource:

Search - Apple Community 

Quick Fix Actions:

 For Apple Silicon computers, use Disk Utility to erase a Mac.

For Apple Intel computers, use Disk Utility to erase an Intel-based Mac, then reinstall macOS.

Always make a Time Machine backup before proceeding.

 Migrate only the user account, not the entire system.

 Reinstall only the necessary applications from the Apple App Store or directly from the developers.