Mohanad, you are welcome. In the future when you post try to limit your posts to a single specific question where possible. Always include the Oracle version you are interested in since Oracle internal behavior has changed over the releases. Also be aware that in many cases the manuals reflect how Oracle normally does things but now how something is always done. For example with 11.2 instead of using the redo log buffer your session may well be writing change data to a private redo log stream. The basic idea is the same but the internal technical details differ on how Oracle handles the redo buffering. When you ask several unrelated questions at once you make it difficult for people to respond in a useful manner plus if you have followup questions the thread can become very difficult to follow.
If you have read some material on a topic and do now understand it or think it conflicts with something else you read that when you post your question reference the manual, chapter, and section topic in your post. This will first identify to readers you have made an effect to understand the material and it will also help the readers determine the appropriate level of detail to reply with.