In this context, "As with" means "In the same way that" or "The same as". It is saying that the istream uses a buffer to communicate with the operating system and that that is the same way that the ostream works.
If you are going to ask about the meaning of words and phrases in this forum, it's important to get the construction of the sentence right. Say "What does "XXXXXX" mean?"
Student or Learner