What is the best way to create a crash report on iOS and Android devices?

I want to create a crash report when my game has crashed on iOS and Android. The problem is that I don’t know how to create the crash report when an unhandled exception occurs. In addition, I can not execute additional code in my Update method after the unhandled exception occurred because the exception already leads to the crash.
How can I create a crash report when my game crashes on iOS and Android? Is it possible to send the crash report immediately to a server when an exception occurs?

Yes, it is super easy with App Center.

Does App Center send the crash report immediately after my game has crashed to my App Center account or does it send the crash report when the player starts my game the next time after the crash?

It sends it the next time the app is started. I’m pretty sure that’s how crashalytics etc. work too.