4. What is the difference between a complex character and a flat character?

The complexity of a character determines the dimensions of a character. Eg, rather than executing his underlings for failure he could say 'Well, that sucks but I can see you tried' at least some of the time. Eg, if we were to flesh out the villain we now learn his life's story, what he likes to do in his free time, who is friends with, what about him isn't completely irredeemable, also he would behave in a more realistic manner.

Three dimensional: When a character actually starts looking like a real person.He'll execute his minions for failure, gloat, monologue, and die cursing the hero. The villain is all about evil because that's his thing. You do get to learn things about them, but they have a role and stick to it.

'Mad scientist', 'Villain', 'power hungry politician', etc. You don't get to know anything else about her: who she likes, what she wants, how is her life like. Eg, Alice the florist, who does nothing in the story besides saying hi and selling flowers.

One dimensional: Background character with zero personality.

It refers to how complex the character is and how much they resemble an actual person.

4. What is the difference between a complex character and a flat character?