Salamo'alaikom, I've read some answers about naskh and it seems that it is the case of surat al Ahzab, which was as long as surat al Baqara but then some verses were abrogated. I'm not understanding this thing in terms of preservation of Quran. What does it mean that Quran has been preserved till now when there is this concept of naskh? Can someone clarify as I don't understand it quite well?
Qur'an is the word of Allah (ﷻ) and it is what He decides it to be. If He decides to abrogate a portion, that doesn't mean that portion is "missing" because He does not intend it to be part of the Book ultimately. So it doesn't contradict preservation nor does it count as corruption.
Once the Qur'an was all revealed and Angel Jibra'il (peace be upon him) and the Prophet Muhammad (ﷺ) reviewed it and it was understood that this is going to be the Qur'an with us forever now until the end of time, the Qur'an has not changed one bit. No accidental changes, no intentional changes could ever touch the Qur'an. It is guarded by Allah (ﷻ).
Compare this to other books in which changes were made by humans that were not authorized by God, in which portions were added or deleted when God did not authorize it. So these have truly been changed. (See Sources of Change in Previous Books)
Let me know if further clarification is needed. Wa Alaikum As Salam.