The (Read-Only Memory) refers to the proprietary operating system firmware that comes pre-installed on your device when it leaves the factory. It includes the Android operating system (typically Android 9 or 10 with Nubia’s custom UI), the kernel, the device drivers, and essential system applications.
ZTE does not always host older ROMs publicly, but they offer downloads via their Nubia community forum. Visit ui.nubia.com or the official Nubia Red Magic support page (since Nubia Z series is part of that ecosystem). Official ZTE Nubia Z20 NX627J Stock Rom