Those firms come from the same Silicon Valley stables as Alphabet, Apple. 

= the same Silicon Valley stables as Alphabet, Aplle are.

= the same Silicon Valley stables as are Alphabet, Aplle.

They are looking at a combined valuation of more than a third of a trillion dollars - roughly, as it happens, the same as that which resulted from the IPOs of Facebook and Google.

破折号后没有省略成分。后面为roughly the same as sth. 即形容词短语，对 a combined valuation of more than a third of a trillion dollars 作补充说明。其中，as it happens (碰巧）为插入语分句。that which=what.