You're looking for information on unsigned IPSW restore tools that are free. Here's some content that might be helpful:
Introduction
Safe free tools (verified):
For specific older devices or technical users, the following tools are the most recognized: Downgrade and dualboot status of almost all iOS devices
Pro: Extremely easy.
Con: Tethered booting is annoying for daily drivers. unsigned ipsw restore tool free
SEP/Baseband Compatibility: Even with blobs, the latest Secure Enclave Processor (SEP) and Baseband firmware from the currently signed iOS must be compatible with the unsigned version you are targeting, or the restore will fail.
Several free tools exist, but none can bypass Apple’s signature check on modern devices (A12+ or iPhone XS/XR and later) without vulnerabilities like SHSH blobs (saved cryptographic signatures when the IPSW was still signed). You're looking for information on unsigned IPSW restore
Key Features: