The unsubscribe_detected_at field is set when RevenueCat detects a user unsubscribed (in other words, opted out of renewal) and is unset (set to null) when a user resubscribes (in other words, opted in to automatic renewals.) RevenueCat will detect unsubscribes much quicker if you have platform server notifications enabled. Without platform server notifications, RevenueCat relies on polling Apple/Google/Stripe, which introduces longer delays.
S3 exports are generated daily, so they’re more of a snapshot of the state of your subscribers. It’s possible for users to subscribe/unsubscribe multiple times in a day, but the state will be captured once per day for the S3 export. So what you see in Google Play Console and Stripe might not match exactly with what’s in the S3 exports if the user subscribed/unsubscribed since the last export.