What happens when you send a message? Does it spam to all platforms? Does everyone have to declare a preferred app to receive? Does it check your previous commu

What happens when you send a message? Does it spam to all platforms? Does everyone have to declare a preferred app to receive? Does it check your previous communication with that contact to see which platform has the best response rate? Just asking, it doesnt seem like it would be that difficult. In fact im sure this used to exist when it was just msn, icq etc.