https://naruto.fandom.com/wiki/Orochimaru#Chakra_and_Physical_Prowess
It says: "He can absorb chakra through physical contact, which also grants him their skills and knowledge". And refers to chapter 593, page 16, but there is nothing about it in that chapter.
Maybe after that, in some chapter, it is said that that moment Orochimaru absorbed the jutsus and knowledge of Kabuto? Idr it happening tbh.