wa alaykum salam wa rahmatullahi wa barakatuh
I would firstly like to point out that the "Sunnah" is the prayers at time with the Fardh. There is no Fardh, Sunnah and Nafil. The nafil are like prayers such as the 2 rakat when you enter a Masjid, the 2 rakat when you do wudhu. You can pray as many nafil prayers as you like. They have nothing to do with the 5 obligatory prayers. Rather, just the sunnah prayers are in the authentic teachings of Islam.
Some people they do extra prayers, but this is the correct way as below:
Fajr, 2 Sunnah 2 Fardh
Zohr, 2 Sunnah 2 Sunnah 4 Fardh 2 Sunnah
Asr , 4 Fardh
Maghrib 3 fardh, 2 Sunnah
Isha, 4 Fardh, 2 Sunnah , then the Witr, which can be 1, 3 etc
This is the way our units of prayer are according to the Quran and authentic hadith.
cuz i see many people just pray farz and they are done
The Prophet

would pray the Sunnah prayers at home. It is better to pray some of them at home, rather than in the masjid. Its not wrong to pray them straight after the prayer in the Masjid, but it is nice if you leave them so you can pray at home.
It was narrated from Ibn ‘Umar that the Prophet (peace and blessings of Allaah be upon him) said: “Perform some of your prayers in your houses and do not make them graves.” (Narrated by al-Bukhaari, 422; Muslim, 777)
This is from Islamqa:
What The excellence of optional Prayers?
Praise be to Allaah.
1. The mother of the faithfull Umm Habibah Ramilah bint Abu Sufyan Radhiallahu ‘anhu narrated that she heard the Prophet Shalallahu ‘alaihi wa Salam saying: “A house will be built in paradise for every Muslim who offers twelve Rak’ah of optional salah other than the obligatory salah in day and night, to seek pleasure of Allah.” Narrated by Muslim.
Note : these extra 12 rakaats of optional prayers, according to Imam Nasaai and Tirmithi are : 2 rakaats before the obligatory prayer of Early morning (fajr), and 4 rakaats before (Duhr) and 2 rakaats after it ,and 2 rakaats after the obligatory of early evening prayer (magrib) and 2 rakaats after the obligatory prayer of night (isha) prayer.
2. Abu Huraira Radhiallahu ‘anhu said : “My Khalil (friend) (the Prophet Shalallahu ‘alaihi wa Salam) advised me to observe three things and I shall not leave them till I die: 1. “To observe Saum (fasting) three days every (lunar) month; 2.” To offer the Duha prayer; 3. To offer Witr prayer before sleeping.” Naratted by Bukhari and Muslim.
3. Aisha radhiyallahu ‘anha narrated that the Prophet Shalallahu ‘alaihi wa Salam said : “The two rakaats before dawn (fajr) prayer are better than the word and all that it contains.” Narrated by Muslim.
4. Aisha radhiyallahu ‘anha narrated that the Prophet Shalallahu ‘alaihi wa Salam was never more regular and strict in offering any Nawafil than the two rakaats (sunnah before) dawn (fajr) prayer. Narrated by Al-Bukhari and Muslim.
5. Umm Habiba radhiyallahu ‘anha narrated that the Prophet Shalallahu ‘alaihi wa Salam said : “Whoever, observes the practice of offering four rak’ahs before Duhr prayer and four rak’ahs after it. Allah will shield him against the fire of hell.” Narrated bay Abu Daud and At-Tirmithi.
6. One should make Witr as the last salah (prayer) (at night).
Abdullah bin Umar Radhiallahu ‘anhu narrated thet the Prophet Shalallahu ‘alaihi wa Salam said : “Make Witr as your last salah (prayer) at night.” Narrated by Al-Bukhari and Muslim.
7. If anyone enters a mosque, he should offer two raka’ah before sitting.
Abu Qatada Radhiallahu ‘anhu narrated that the Prophet Shalallahu ‘alaihi wa Salam said : “If any one of you enters a mosque, he should pray two raka’ats before sitting.” Narrated by Al-Bukhari and Muslim.
8. Abu Huraira Radhiallahu ‘anhu narrated that Allah’s Messenger Shalallahu ‘alaihi wa Salam said to Bilal Radhiallahu ‘anhu “Tell me about the most hopeful act (i.e one which you deem the most rewarding with Allah) you have done since your acceptance of Islam because I heard the sound of the steps of your shoes in front of me in paradise.” Bilal said : “I do not consider any act more hopeful than that whenever I make ablution (wudoo) at any time of night or day, I offer salah (prayer) for as long as was destined for me to offer.” Narrated by Al-bukhari and Muslim.