1.When the athan calles for prayer, can i do nafilah exactly when the athan is calling for prayer? U have seen people who pray nafil when the athan is upon calling (has not yet finished calling for prayer)
It is better and more correct to listen to the adhan, repeat after the muadhdhin, then make the legislated duaas, and then perform your nafilah.
Praying during the adhan is valid, but you miss the Sunnah of responding to the adhan.
The Prophet ﷺ taught that the Muslim should:
Repeat after the muadhdhin
Hadith:
The Prophet ﷺ said: "When you hear the muadhdhin, say what he says."
Arabic:
«إذا سمعتم المؤذن فقولوا مثل ما يقول»
Reference: Sahih Muslim: 383
This is a clear instruction, and responding to the adhan is a Sunnah action.
Send salaat upon the Prophet ﷺ after the adhan
The Prophet ﷺ said: "When you hear the muadhdhin, say what he says, then send salaat upon me, for whoever sends salaat upon me once, Allah will send salaat upon him ten times."
Arabic:
«إذا سمعتم المؤذن فقولوا مثل ما يقول، ثم صلّوا عليّ، فإنه من صلّى عليّ صلاة صلى الله عليه بها عشراً»
Reference: Sahih Muslim: 383
Make the famous duaa after the adhan
The Prophet ﷺ said: "After the adhan, say: O Allah, Lord of this perfect call... give Muhammad the praiseworthy station."
Arabic:
«اللهم رب هذه الدعوة التامة…»
Reference: Sahih Al Bukhari: 614
All of these Sunnah actions are done after hearing the adhan.