You grabbed a lightweight "Recovery Only" DMG instead of a full "macOS Restore" DMG. Fix: Search for macOS Restore IPSW (ironically, Apple Silicon Macs sometimes prefer IPSW over DMG). Download the full 12-14GB IPSW file.
file for public download on their official support or download pages. Why People Look for the DMG Most users seek a .dmg file because they are trying to: Install on Older macOS Versions : The current Mac App Store
