I don't understand why you've only had this situation for a couple of weeks, when the feature has been on all iPhones for over a year. The reason you didn't have it 10 years ago is that iCloud has only been around for a couple of years, so with no iCloud this feature wasn't available. If you have separate IDs and use family sharing you can still share a calendar. For Contacts, turn off the shared ID and you will be prompted to keep contacts on the phone. When you create a new ID those contacts will be associated with the new ID. Does everyone really need each other's contacts? I have 1100, my wife has 300, and there isn't that much overlap.
Anyway, you have a feature enabled that allows you to answer a call on any device. That's why all devices sharing an Apple ID ring. It's a great feature; if I'm at my Mac I can answer a phone call from it, rather than pull out my phone. Likewise for my iPad. You can turn off this feature; go to Settings/phone and turn off Calls on other devices. Or just turn off some devices. Do this on each device.