Kaboom-y numbers!

Here is a new type of puzzle poem I made. Enjoy!

Note: The title is named like that for a reason.

I have a number in mind, it is super cool,

But there’s something you need to know, or you will find it like a fool.

The greatness of this number, the primes you need to understand,

Prime number 21 is the greatest, here’s why it’s the "great man":

A mirror, not the one made of glass,

Prime number 12 is the opposite, of the above you learned in class.

Here’s another one, you’re sure to appear in shock,

Multiply it’s digits, and you will get 21, what a surprise in stock!

Now, it has another name, from famous science and math geeks,

In fact, if in front of the big metal and glass box,

A geek there is the name of the number I speak.

Apply a code, another prime, you know,

Do with the first provers too,

And you get the special code.

This code very special,

Unlock a safe for hidden secrets

For the secrets of the universe,

Wait, I’m wrong, it’s a number, now I’ll speak again in verse!

ughhhh…

What do you think is the code to the safe?

BONUS: Tick only to the first who finds the special "secrets of the universe" number.

Note: Only use the now encoded words in order: the geek, the H guy, (Spoiler revealed), and the last one.

Hint #1:

Hint #2 (Super Hint):

Hint #3 (Spoiler Hint):

Hint #4 ( Just realised this):

Hint #5:

By the way, no more hints from here, so I am hoping someone would solve it 😉