If you have a signed IPSW file and want to install it manually: iPhone 8 (GSM) IPSW Download - iClarified
You can technically download the iOS 12.1 IPSW file for your iPhone 8 (GSM) Go to product viewer dialog for this item. Download iOS 12.1 for iPhone 8 (GSM)
You cannot downgrade from a newer version (like iOS 16) to 12.1. 📥 How to Download (Archive Only) If you have a signed IPSW file and
Apple uses a "signing" system to control which iOS versions can be installed. iOS 12.1 is currently unsigned. Download iOS 12.1 for iPhone 8 (GSM)