rationalfrummy, practically any Meforash Al Hatorah discusses why he married two sisters.
As LanderTalmid said, the Avos studied what was passed down through the generations and what was added to it. They studied the secrets of the Torah. They also had a Mesora on the Halachos, as Chazal say about Noach.
Don’t confuse Torah with Sefer Torah. Chazal say that the creation started with the ?, and the Torah started with the ?. Torah means Hashem’s instructions. That is what they studied.