A firmware is an IPSW file or OTA update ZIP file that contains everything needed to install the core operating system of a device.
To check for iOS, iPod, Apple TV, and HomePod mini updates, iTunes, Finder, Apple Configurator, and Apple Devices contact the main manifest at https://s.mzstatic.com/version. This manifest also contains carrier bundles. There are separate manifests for macOS, bridgeOS, and visionOS.
The first-generation Apple TV also uses a firmware manifest, found at https://mesu.apple.com/version.xml. It uses separate files, rather than consolidating them into an IPSW archive.
The original use of the version manifest was to notify the user of iPod firmware updates. The notification directs the user to the Apple Support website to download iPod Updater. With iTunes 7.0, iPod Updater functionality was merged into iTunes, using IPSW files to hold the firmware. iTunes 7.3 extends the manifest and IPSW file format to support the iPhone.
There is no public manifest of IPSWs for any Apple Watch, HomePod (other than HomePod mini), or Apple TV 4K and later. These devices exclusively use OTA updates, and can be recovered using recoveryOS. Some IPSW files have been discovered for these devices through various means, but lack of a user-accessible Lightning or USB-C port renders it difficult to make use of them.
