Treat the category as a starting point
The storage chart groups data into categories. A category label does not tell you that every included file is expendable, and it is not necessarily the name of a folder you can remove. Your first aim is to locate the source of growth rather than force the chart to reach an arbitrary target.
Open System Settings → General → Storage and wait for the calculation. Note the available space, the category size and the time. Compare later measurements in the same view and on the same disk.
Check what changed before the number grew
Think about recent developer builds, large application updates, device backups and media work. This gives you concrete locations to investigate. A Mac used for development has a different storage pattern from one used mainly for browsing and photos.
Check whether downloads or exports are still running. Storage that is actively changing is difficult to diagnose from a single screenshot. Finishing the work and measuring again can prevent you from deleting files to compensate for an ongoing job.
Review specific kinds of data, one at a time
For local iPhone backups, use Finder’s backup management screen to identify old devices and dates. For Xcode, distinguish rebuildable DerivedData from source projects and release archives. For a browser problem, use that browser’s own data controls.
Do not delete the whole Library or a system directory because a search result lists a large path. A folder can contain settings, working documents and generated files together. When you cannot explain what an item is for, leave it out of the first cleanup.
Free useful space without chasing the label
You may be able to make room by archiving old videos, removing verified duplicate downloads or deleting an installer after confirming its app is installed. Those actions can help even if the System Data label does not immediately shrink.
macOS also manages some temporary data itself when space is needed. Removing a frequently regenerated cache may produce a short-lived change. Judge the result by whether you now have room for the intended update, download or project, not just by one coloured section of the chart.
Record the change and scan only known locations
After a targeted cleanup, review the Trash and compare storage again after the display updates. If a particular application keeps creating large files, investigate its settings or support information instead of running the same cleanup daily.
Storage Cleaner: Disk & Cache can help review large files and cache/developer categories in folders you authorize. Scanning is free; cleaning requires a purchase. It is a way to inspect candidates, not a promise that every byte shown as System Data can be removed.