Memory is RAM ( Random Access Memory ) and is filled up and empties as the Computer needs Applications to load and unload.
Drive Capacity is a totally different matter and when the computer warns the user " Running Low on Space " that is time to action this before the computer can and may cease to Boot-up at ALL
It is suggested to keep at least 15% to 20% of the Drive Total Capacity as Empty Space for good operations of the computer.
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OmniDiskSweeper Safe to use
GrandPerspective
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The final word from Apple on Managing the " Other/ System Data “ Category
Other / System Data: 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.