If they're on different phones, they could be on different bands, different towers, and different paths, one or more of which could be impacted by whatever the underlying problem is. iPhone vs Android would be the most blatant tell that something like this is at fault, but it could also be different configurations from different stores causing them to interact with the cellular network in different ways.

Something like a routing configuration, BGP failure, or underlying network misconfiguration would cause seemingly bizarre results with some phones working and some not with no obvious correlation. Compare Access Point Names under Mobile Settings on android, and whatever the equivalent is on iPhones, and check things like whether 5G allowed and data roaming is enabled.

If it's a cyber attack of some sort, then there's all sorts of different attack vectors that would cause these outcomes.

My wife and I are on the same Verizon family plan. One of us can be down while the other is fine, then 30 minutes later it's the opposite. It's been like that all day.

Same here, except that when here (central-western NJ) when someone "recovers" here we go from SOS to a few bars but no LTE or 5G indicator. Yikes.