From blog.elcomsoft.com
The keychain
The HomePod contained a copy of the keychain. Once we loaded the file into Elcomsoft Phone Viewer, we were able to see the keychain items available.What we saw was a long list of Wi-Fi passwords, which contained passwords to pretty much all Wi-Fi access points stored in the iPhone on the same Apple ID. We also found a list of tokens including an Apple account token. We have not looked into this any further.