Both are good mate!

Both would work just fine. I usually say. "Okay, okay, now I understand." or "Okay, I understand what you are talking about now."

Either is great! If you want to be extra specific, you can choose one based on how much time has passed since you learned/"understood". If someone just explained something, or you literally just in this moment understood something, "now I understand" is slightly more common. But if you understood last week/month and you were telling someone you know better in this moment, "I understand now" is more appropriate. But really, either is fine in most situations.

Both are correct! They mean the same thing :)

You can also say “I now understand”.