System data is taking half my drive storage - how do I reduce this?

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The final word from Apple on Managing the " Other " Category

• 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.

See:

What is “Other” storage on a Mac, and how… - Apple Community

Now Other is called System Data

• If that's the space macOS is taking alone, there is nothing you can do about it.
• If that includes the 'Other' folder, see https://www.imore.com/how-find-and-remove-files-other-category-mac.

If you’re on a 128 GB storage configuration as I might suspect, you’re unfortunately not going to avoid managing your storage.