User Profile

I am learning about swiftui. I want my app to be able to send and receive a small amount of data to another app on another device. That device will include andriod as well as Apple. Since I am new to SwiftUI and iphone app development, I am not sure what to use to accomplish this. I thought that notifications would work well because the client app doesn't need to be in a run state or even in the foreground. However I decided I want to work with cross-platforms. I looked into Firebase but still have a lot of research to do. What can I use to send and receive data in a cross-platforms?