iPhone device backups are stored at ~/Library/Application Support/MobileSync/Backup/
The replies in that other Discussion are basically correct, and no iPhone software update files on your Mac will remain once they are installed on your iPhone. That's the reason you can't find ~/Library/iTunes/iPhone Software Updates. It's gone.
If you want to restore your iPhone to factory settings, then read and follow Restore your iPhone, iPad, or iPod to factory settings .... It also describes how to subsequently restore its content from an iTunes backup. I don't blame you for wanting to know just where it's stored on your Mac, but it's unnecessary knowledge.