: A "custom" IPSW file specifically from Unlock Hub (e.g., "Unlock Hub iOS 15.7.5 iPhone 7 Plus IPSW") is supposedly required.
For a safe and legitimate unlock, consider these verified methods:
An IPSW file is essentially a compressed archive (ZIP format) containing the root filesystem and bootloader images for iOS devices. A "Custom IPSW" is created by modifying specific files within this archive to alter the behavior of the boot process.