Before using the iPhone Firmware Flash Tool, make sure to:
Flashing firmware manually requires an IPSW file. Apple must still digitally sign these files for the installation to succeed. Visit IPSW.me
This site provides download links for iPhone, iPad, Mac, Apple TV, HomePod, iPod touch, and Apple Watch firmware across every released version. Each listing clearly indicates whether the firmware is Signed (can be restored via standard tools) or Unsigned (requires SHSH blobs for restoration).
Completely free, automatically matches available IPSW files, offers an "Easy Flash" mode. Cons: Windows only, heavily dependent on network stability.
“I’m telling you, the official restore keeps throwing Error 4013,” Elias muttered, rubbing his bloodshot eyes. He reached for his lukewarm coffee. “It’s a NAND issue, or the handshake is failing.”