When you open an online-only file, it downloads to your device and becomes a locally available file. You could open a locally available file anytime, even without Internet access. This is by default, we could change the file back to online only. Just right-click the file and select “Free up space.”
With Storage Sense turned on, these files will become online-only files after the time period you've selected.
For more information, you could refer to:
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The requirement is to make the file always online.