Restoring Mac mini

Restart with the Command and R keys held down, use the Disk Utility to erase the internal drive, and install a fresh OS.

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I, personally, used USB boot drive (I have saved 10.12 ~ 10.15 each OS individually) for this kind of process.

How to create a bootable installer for macOS

You can boot directly from USB drive (by holding "option" key) and access "Disk Utilities" to erase and re-install 10.15.