Create a simple app for IOS and Android to send notifications to a group

We are a group of people who want to notify each other using an app. Just a button in an app that when we press, it sends a notification to other people’s phone.

– Some people in this group have IOS and some Andriod.
– It might also be possible to make this on windows. (when you push the button, people will be notified)
– If you have a service like Zapier or something else in mind, let me know about that too

Example: I open the app, click the button, Person A with IOS, and Person B with Android see a notification with sound on their phone.

Please explain to me how you’re planning to do this task.