checkout the apple-ios tag too it has lots of clarifications (and solutions at times).
on 2) Backup: got better in recent Android versions (past A15) and /e/OS 4.0 too has improvements - but it isn’t care-free. Messengers often handle their own backup that you need to pick up in the backup config
on 3) Synchronizing a Calendar: you can subscribe to any .ics link a iCloud user shares with you. For the other direction see this thread. You can’t all write to the same calendar, but you can share each others calendar and the Calender app will aggregate