Minify JSON online and see the bytes you saved

It depends entirely on how deeply nested the document is, because indentation is what gets removed. Deeply nested formatted JSON often drops by half. A flat document with short keys may barely change.

It rarely matters on its own, because gzip and brotli already compress repeated whitespace very well. Minify when the destination has a size or format limit, such as an environment variable or a query string.

No. The document is parsed and re-serialised without whitespace. Every key, value and type is preserved.

Yes, run the output through the JSON beautifier. Whitespace is the only thing removed, so a readable shape comes straight back.

Because the content, not the formatting, is the bulk of it. Long repeated key names in a large array are usually the cause. Shortening keys or moving to a columnar shape helps far more than minifying.