Given Node-RED is intended as a low-code programming tool, hand-writing the flow JSON has never really been a requirement - certainly not one I can recall being brought up in the community. It's certainly possible to generate flow JSON via other means as the format is not hugely complicated once you know the format.

It's an array of objects, each representing a 'thing' in the flow. Each object has a unqiue id (which can be any randomly generated value), and other meta data that defines what the object is and what its configuration is.

Given most nodes have been implemented with a graphical UI in mind for configuring the nodes, you'd have to refer to individual nodes to understand what node-specific properties they require.