IPhone : Can anyone provide hint or advice regarding IOS apps that I am willing to develop?

I am thinking to develop IOS apps in Swift. This app would somehow similar to "Notes" apps on iPhone that sync across all apple device.

The concept would be same. Let's say someone download my apps and write any "text", the other can download same apps and they both can see same text-syncronised each other's text.

How is this possible? Which database would help me to develop this? Please pass me any similar tutorial or reference that can help me to study the mechanism.

Any advice or suggestion would be appreciate.

Thanks, Swikar