Original post by TawheedTo answer the second part of your question, while we do take narrations from companions, and accept a lot of what is in Bukhari, Muslim etc, we tend to emphasise the following chain of narrators to Rasulullah s.a.w:
1. Ali ibn Abi Talib a.s - A hadith narrated from him, to Rasullulah s.a.w is the strongest.
2. Hasan and Hussain ibn Ali a.s - a hadith narrated from them from Ali a.s is the strongest link to Ali a.s
4. Ali ibn Hussain a.s to his father, Hussain ibn Ali a.s - He was the great Grandson of Muhammed s.a.w, and recognised as a scholar of his time by both shia and sunni's.
5. Muhammed Al Baqir a.s - another highly revered scholar, and direct descendent of Muhammed s.a.w by both shia's and sunni's. Dhahabi reports an incident when Rasulullah s.a.w, asks jabir ibn Abdillah (radiyaullahu anhu) to send his salams to him. Since Jabir (r.a) lived a long time, he managed to do this according to reports.
6. Jaffer As Sadiq a.s - the son of Muhammed Al Baqir a.s, and highly revered by both shia's and sunni's.
So to give you an example, when fulan bin fulan, reports from fulan, that Jaffer as Sadiq a.s said : Rasulullah s.a.w said "
The chain of narrators is: Jaffer as Sadiq a.s, from his father [Al Baqir], from his father [Ali ibn Hussain], from his father [Hussain ibn Ali a.s/Hasan ibn Ali a.s, the grandchildren of Rasulullah and the leaders of the youths of paradise], from Ali ibn Abi Talib a.s, from Rasullulah s.a.w].
When you have narrations where Rasullulah s.a.w is not explicitly mentioned, for example, in a fiqh law, or anything of any subject matter, for example:
Narrated Ali ibn Hussain: " x is harram, and y is considered makruh".
Even if it does not explicitly state that 'Rasullulah said 'x is harram and y is makruh', the original source of this is from Rasulullah s.a.w.
The chain is Ali ibn Hussain , from his father, Hussain ibn Ali a.s [and / to Hasan Ibn Ali a.s], from Ali ibn Abi Talib a.s, from Rasulullah s.a.w So ultimately, every ahadith goes back to Rasullah s.a.w, from either the companions, or, [as we prefer it], from his Ahlulbayt asws, who we believe preserved his true sunnah.
Rasulullah s.a.w had stated he left behind two weighty things, The Quran, and his Ahlulbayt. We follow the Ahlulbayt asws to get to the Sunnah of Muhammed s.a.w, since we believe, by the will and ordination of Allah azwj, it was them upon whom the task of preserving his sunnah was placed.
Remember, the big dispute and the reason why Islam has split into so many groups is not because we don't recognise the need to follow the Sunnah of Muhammed s.a.w, but rather, where to get his sunnah from. Thus, we believe, Rasulullah s.a.w clearly hi-lighted from who we can obtain the purest stream , the purest form of his sunnah - his Ahlulbayt.