You can either say "I get it," "I got it," or "got it." You would not say "get it" by itself. The difference between get and got is just tense (present vs past), but in answering that you understand something, you could use whatever comes more naturally. Most people I know usually say "I get it" or "got it".

Both are fine if it is something someone has just told you.

You probably saw this in a movie where a man says 'get it?' to make sure somebody understands what he said or understands the complexity of a situation. We could say 'got it' as the answer or 'Yes, I get it' In a movie a man could also say 'got it?' to make sure the other person understands how serious he is about something, he would usually do this to intimidate the person so he would use an intimidating or menacing tone. And yes, they can be used as two word questions.

U can say I got it if u understood the qjestion also

Get it ,means u understood the question and got it means like I caught the ball. .