Which is correct and why:

Who am I?


Who I am?


"Who am I?" is correct if you are asking a direct question.

In indirect speech you might say something like "I wonder who I am."


~comes back to edit~

I forgot you asked "why".
The verb "to be" doesn't use any auxiliaries in the present simple, so in order to make a question you only need to invert the position of the subject and the verb.
In an affirmative sentence, you say:
"I am atm."

In a question, you turn "I am" into "am I", so you have:
"Who am I?" (for a wh-question)
"Am I atm?" (for a yes-no question)

I am
You are
He/she/it is
We/you/they are

Am I?
Are you?
Is he/she/it?
Are we/you/they?

Hope it helps.
Thank you, Mariam.

When a company uses a question like "who are we" to introduce itself (e.g., in a corporate profile), will it be considered a direct or indirect question? In this case, although the company is asking the question, but the answer is aimed at someone else.

Teachers: We supply a list of EFL job vacancies
You're welcome, atm. Emotion: smile

I've seen those "who we are" pages in many websites. The company is not really asking the reader a question; rather, they are saying "We're going to tell you who we are" or "We're going to tell you something about us" ("Who are we?" would be a direct question).
They are similar to pages such as "where we are", "what we do/offer".
The form, as you said, is that of an indirect question, but you're not really being asked anything.

What is the negative interrogation of "I am"? Need your help.
am I not?
Students: We have free audio pronunciation exercises.
Thank you! So, it's a little different from " aren't you?" " isn't he?" etc.