السلام عليكم
From what little I have understood, Sunnah cannot be something that scholars have had to use the Quran or the Akhbar to prove. Instead, it has to be something which scholars agree that it has made its way to us through Ijma and Tawatur. I think Salah is a good example. The timings of Salah and how to pray, we get all that from the society. We don’t have to look at the Quran or the Akhbar. Children who don’t understand the Quran and haven’t read a single report regarding prayer pray just like the rest of the society. On the other hand, whenever someone tries to teach young women that they are supposed to cover their hair, the Quran is always referred to (Surah Al-Ahzab, usually). This is because you cannot say that this has reached us from Ijma and Tawatur and scholars haven’t ever claimed that either as far as I know (which is quite little, though 😅) They have always brought the Quran or the Akhbar as evidence to prove that women must cover the entirety of their body. If you are interested in Ghamidi Sahab’s reasoning, you can read about what can be termed “Sunnah” (in his opinion) in his book, Meezan, in مبادی تدبر سنت:
https://www.javedahmedghamidi.org/#!/mizan
Also, the Sunnah cannot be determined by what people of any time or place did until or unless it can be proven that the Prophet صلى الله عليه وسلم himself made it a part of religion because he is the only source of religion for us.
Regarding whether everything that is Sunnah is mandatory, someone can correct me if I am wrong but I don’t think so. For example, sacrificing an animal is a Sunnah but it is not mandatory except sometimes as compensation.
I hope that helps and feel free to question further.