What is NDJSON?
NDJSON is one JSON object per line, and nothing wrapping them. That single change makes it streamable - a program can handle a line at a time instead of holding the whole array in memory - which is why logs, data exports and machine-learning datasets ship in it.
What is CSV?
CSV is the least glamorous and most durable data format there is - rows of values separated by commas, readable by every spreadsheet and database ever written. Its subtleties are all about quoting: a value containing a comma has to be wrapped, and a quote inside that has to be doubled.