It is said that each time you say peace be upon the prophet of Islam, this message is transmitted to him, he hears it and he respond to your Salam. If potentially, thousands of Muslims are saying this at the same time, how this process would work ?
This is not a question we have an answer for.
After death,you basically go to a completely different life.. which could not follow our mind-limited physical laws..
It's all from the knowledge of 'ghayb which we shouldn't put our noses in.
Just like asking " How can Jibreel have 600 wings ?" or " How can Allah talk to us all on the day of judgment simultaneously?"
This is a red line which you shouldnt cross.