What is NDJSON?

One JSON object per line, with no wrapping array. That small change is the whole point: a program can read a record at a time instead of parsing gigabytes into memory first, which is why log pipelines, bulk importers and search indexes all speak it.

What is XML?

Verbose, strict, and still holding up enormous amounts of the world - government feeds, financial messaging, publishing, office documents. JSON took the web from it, but anywhere with schemas and validators and twenty years of history, XML is what's on the wire.