“I got it” and “I get it” can be used to say “I understand”. Personally, I do not use them interchangeably. I would use “I got it” to convey, for example, that I heard what you just said, but “I get it” to communicate that I understand something more important. “I got it” can also used in different ways. For example, you could say it if you were offering to pay for a shared meal, or describing that that you just got the new iPhone that was being discussed, etc

Thank you Dr. B

We do use them interchangeably although it probably isn't proper English. "Get" is present tense and "got "is past tense but we routinely use them in the present to let someone know that we understand something.