That's what the website missing indeed. We have a few words about that in docs, but it's still not enough https://jitsu.com/docs/internals/jitsu-server#mapping-step
Overall, Jitsu tries to decompose (aka flatten) JSON as deep as possible. E.g. {a: {b:1, c:2}} will become a_b=1, a_c=2. If column is missing, it will be created. We don't decompose arrays so far