If you want to know everything about how to "Tailor the VoiceOver experience in your data-rich apps" with the Accessibility Custom Content API, there is a WWDC21 session.

https://developer.apple.com/videos/play/wwdc2021/10121/

When implementing accessibilityCustomContent, for any supplementary information, it returns an array that VoiceOver will announce in that given order. The value of the AXCustomContent first, then the label.

Users can configure in VoiceOver's verbosity settings if it should say that there's more content available, or play a sound hinting that there is, or simply do nothing. So it should really be optional content as users might miss it.