[Ali Imran]

All the time i tot their Koran was fully written and sent to their muh in one day
But no, it needs 23 years of their prophet to tell his people and the people to write it down after his death
How to know if no thing is being altered ?

Yes, the revelation that took 23 years is a miracle, if you know the facts. It will take me quite a while to explain why that is a miracle.

How to know if nothing is being altered? The hundreds of Hafiz, people who memorized everything.