• The Rif
• The Ran [or Nimukei Yosef] on the Rif
And the Rosh
But most yeshivah bachurs do not realize that generally speaking the Talmud Text embedded within the Rosh is actually the text of the Rif. And therefore, that the Rosh essentially is an expanded Rif - as contrasted to an external commentary such as the Ran.
This factoid is better know in the scientific ["Wissenshaft" or Hochmat Yisrae'l] community than in the Yeshivish Community. One of my colleagues - whose PhD is on the Rif - is most aware of this phenomenon
But many are not aware of this. And so how can this awareness matter?
The sefer M'irat Einayim - who may be the premier commentary on Shulchan Aruch * - states that Halachah follows the Rif EXCEPT when Tosafot disagrees. And now you have the bedrock of Ashkneazic methodology in p'saq.
But this SM"A is not really saying anything new or radical! To a great extent the Rosh and the Mordechai incorporated this very premise into their commentaries on the Halachah. It is a pre-supposition that guides them - for the most part.
The Scientific Component here can show us how the Rishonim viewed Shas better than the less disciplined techniques of seeing Rosh as just commenting without a methodology,
The Ran - who is also a major player in P'saq - employs this technique too - albeit to a lesser extent.
The pure, pristine Rif was a bigger player amongst early Rishonim. But later Rishonim and early Acharonim have mostly filtered the Rif through the prisms of Ran, Rosh, and Mordechai. And this factoid shows the technique that is common to the three.
The Rema seems to primarily favour the Mordechai, but a quick review will show Rosh and Ran as major players, too.
EG Tosafot argues for women's right to recite brachot on EG Shofar, Sukkah, Lulav.
But it's the Ran's formulation and authority that is cited by Beth Yosef and Rema as supporting this P'saq.
As we see from this Scientifically informed view, that Shas was used rarely as the direct source of Halachah - rather it was filtered by the Posqim first
I hope this helps
* How the SM"A became a premier commentary on the SA
The SA was seen by the Levush as simply too brief to be a primary Halachic text.
The L'vush added a "qitzur Beth Yosef" approach. For a generation or so the L'vush largely supplanted the SA
The SM"A took the same approach to the SA by supplementing it with a "mini-BY". He became the first major commentary on the SA and restored the SA to primacy over the L'vush. W/O nosei Keilim, the SA might have sank into the background of the Halachic Universe.