It doesn't matter what's inside

Actually, the difference to them is the subject. (With one of "

it

"And the other is"

The

”)

It

" is a

Dummy pronouns

that doesn't need the predecessor. Simply "it doesn't matter" means "just don't pay attention because there's no devil in the details."

The

“However, is a

Demonstrative pronouns

that needs a history.

The,

as demontative pronouns (singular of these) gives a distance from the speaker. - -

The

(This screw, a few meters away from me, doesn't fit the nut I'm holding.)

does

n

does not matter.

In conversation,

It does not matter

is used to signal that the listener has no concerns about the speaker's topic, although the speaker may have had concerns, and that the listener has nothing more to say on the topic.

It does not matter

suggests that the listener has concerns about the topic but is not concerned with what the speaker has just said. Instead, the listener will likely provide their opinion on the subject with a construction like the following:

The

[which means what the speaker just said on the subject]

does not matter. Now let me tell you what is really important.

It could also mean that the listener believes the speaker is wrong about the topic and intends to provide an alternate view or correction.

So use

The

If a contrasting point or opinion needs to be introduced, the

The

(the original speaker's point) as opposed to an implied

these

(Responder's point). Use

it

generally referring to the speaker's point when there is no intention to pursue the subject further.