What is JSONC? Are JSONC and JSON-C different?

There is also jsonc aka "JSON with comments", created by Microsoft and used by Visual Studio Code. The logic for it can be found here, alas without exhaustive specification (though I'd like to be proven wrong on this).

On top of that there is this project with an actual specification which is also called jsonc, but also does far more than just adding comments.

While there definitely is a use for these technologies, some critical thinking is advised. JSON containing comments is not JSON.

JSON-C seems to just be a variation of JSON mainly targeted at C development. I.e., from the open source docs, "JSON-C implements a reference counting object model that allows you to easily construct JSON objects in C, output them as JSON formatted strings and parse JSON formatted strings back into the C representation of JSON objects."ref^1

From the YouTube API perspective (specifically, version 2, not the new version 3), The JSON-C response is just a condensed version of the JSON response (removing "duplicate, irrelevant or easily calculated values").ref^2

Why would the JSON response have "duplicate, irrelevant or easily calculated values" values anyway? Because it is converting the original ATOM XML format directly to JSON in a loseless conversion. You can find out more details here.

However, I would suggest using version 3 of the YouTube Data API. It is much easier to use. =)