Я зрозумів ≈ I have understood. You can do this [understand] only immediately, so it is to finish [understand something, usually all of the thought].
Я розумів ≈ I was understanding. This is understanding while something [are doing] in during time, or you understand something to point of time and after you do not understand for some reason.
Я зрозумів, що він сказав. You understood all after (it can be also after a long time) his talking.
Я розумів, що він сказав. As you see, сказав is not “are doing”, so we take the second part: at least now you do not understand what he said.
For the first part, we must change word сказав to казав: Я зрозумів, що він казав.
- He was talking and you understand him
- You already know or understand the theme before of his talking.
Я зрозумів, що він казав.
- You understood him before he ended talking.
- Same as with сказав, but with a focus that he talking long.