Anike has explained the difference above. To add to her answer: In your question you ask 'what situation to use' these terms. We use these phrases in informal speech, so when talking to a friend or someone in a situation where you are both relaxed and talking in a friendly way. Sometimes people will be offended if you say "Get it?" because they think it sounds quite aggressive/too direct. This varies in different cultures (so in different English speaking countries/organisations/groups of people), but generally you would not use 'get it/got it' in any situation where you want to be more formal (like a job interview for example).

Get it? (do you understand?) / Got it (I understood) is casual speech. In some situations, "Get it?" or "Do/did you get it?" is condescending. Angry adults will scold their children with statements like "You know you are supposed to do your chores on Saturday morning. Do you get it? (or do I have to punish you/ground you until you do get it...)" "Got it" (= I understood, or I received the message) is very casual. I hear it from my teenagers. I don't hear it from adults.

Get it? Is present tense Got it is past tense.

I'd say 'got it' when you understood the idea or proposal.