I have exactly the same question.
I took half a day to figure out how to verify the cert chain in x5c, couldn't make it until I found this question.
It's really bad work as there's nothing described in the official guide https://developer.apple.com/documentation/appstoreservernotifications
Not mentioned in WWDC21 session as well https://developer.apple.com/videos/play/wwdc2021/10174/
southernkasaist
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