The best unsigned IPSW restore tool is FutureRestore + saved blobs for A7–A13 devices. For A5–A11, iPwnder32 gives unlimited unsigned restores without blobs. For A14+ (iPhone 12 and newer) no free tool exists – you cannot restore an unsigned IPSW at all.
Used by developers to bypass some iTunes limitations, though it still requires valid signatures for modern devices. : Best for older devices (iPhone 5s and older). unsigned ipsw restore tool free
To use any tool for an unsigned restore, you generally need for the specific version you want to install. These are digital signatures unique to your specific device (identified by ECID) that "trick" the device into thinking the version is still signed. Top Free & Community Tools The best unsigned IPSW restore tool is FutureRestore