Linking selects via json - where to read about it? What query to google?

Question

JavaScript

Such task, earlier directly did not face, I want to understand.

I google for form select relations json - something is found, but more general. If you know good articles or videos on this topic, please share the link.

I almost don’t understand json, I once watched a video. but in practice it came in handy only now. To penetrate as easily as possible, and to cover this particular topic. I don't even know which way to go. Do not review all the json videos that are on YouTube.