What is IPSW and Why need this file .ipsw ?
1. Because .ipsw is the iOS firmware which you can use to restore, update, or downgrade your iDevices.
2. If you have limited Internet data, backup this file comes in handy as you can just drag and drop it to iTunes when you want to restore, update or downgrade your iPhone or iPad. iTunes will extract local file rather than download it from the Internet.
Where to find this .ipsw file after I downloaded it?
How to Find and Access IPSW Files in macOS (Newer version)
~/Library/iTunes/iPhone Software Updates
How to Find and Access IPSW Files in macOS (Older version)
~/Library/Application Support/iTunes/iPhone Software Updates
How to Find and Access IPSW Files in Windows
Note: Replace USERNAME with your own Windows account username to find it on your own PC
Windows XP (iTunes Desktop version)
\Documents and Settings\username\Application Data\Apple Computer\iTunes\iPhone Software Updates
Windows Vista & Windows 7 (iTunes Desktop version)
\Users\username\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates
Windows 8 & Windows 10 (iTunes Desktop version)
\Users\USERNAME\AppData\Roaming\Apple Computer\iTunes\
Windows 10 (and latest) (iTunes App version)
C:\Users\USERNAME\AppData\Local\Packages\AppleInc.iTunes_nzyj5cx40ttqa\LocalCache\Roaming\Apple Computer\iTunes\iPhone Software Updates