Why do we use "об этом" for 'about it"

Question

In the prepositional case, a phrase "about it", is usually translated as "об этом". Is "этом" in this phrase, a declension from demonstrative pronoun "этот" ? And if that's so, why there is no noun after a demonstrative pronoun?

Answer 1

"about it", is usually translated as "об этом"

Could also be про это. There's even a poem Про это by Mayakovskij.

why there is no noun after a demonstrative pronoun?

Because Это may also be used in place of a noun. Sort of This or That. For example,

Это кажется мне странным. --> It seems strange to me.
Что Вы под этим подразумеваете? --> What do you mean by that?
Что это такое? --> What is this?
etc.

Answer 2

Is "этом" in this phrase [about it, об этом], a declension from demonstrative pronoun "этот" ?

Это is the demonstrative pronoun of neuter gender. Этот is the same pronoun, but of masculine gender, and эта is that of feminine gender. Sometimes the way the three are inflected for case is homonymous.

Here`s an example.

• So what are we going to do about IT? — Ну и что мы с этим будем делать? (Instrumental case of это, neuter gender.)

• So what are we going to do about HIM? — Hу и что мы с этим будем делать? (Instrumental case of этот, masculine gender.)

Same form, different meaning.

And if that's so, why there is no noun after a demonstrative pronoun?

Because in this case this demonstrative pronoun would be self-sufficient as the indirect object of the sentence.