Firmware for iPhones comes in files (iPhone Software). These contain the entire operating system, baseband (for cellular), and restore ramdisk. For the iPhone 6s, each IPSW is device-specific and size ranges from 2.5 GB to 3.8 GB .

So unless you have blobs, accept that iOS 15.8.3 is your only option.

As of late 2024, the iPhone 6s is stuck on the final iOS 15.8.x branch. You generally cannot downgrade to iOS 14 or iOS 13 because Apple has stopped signing those versions. When browsing IPSW sites, look for a green checkmark (signed) or a red cross (unsigned).