Why does it tell me the line and column for JSON?

Because "Unexpected token" on its own is useless in a four thousand line file.

What does minify actually do here?

It removes whitespace and nothing else.

Format and validate code

JSON, HTML, XML, CSS and JavaScript.

Indent

Input

Result

Minifying here only removes whitespace. It will not rename variables or drop unused code, because those need to understand the program and are a build step rather than a text tool.

Questions

Why does it tell me the line and column for JSON?: Because "Unexpected token" on its own is useless in a four thousand line file.
Is my code uploaded?: No.
What does minify actually do here?: It removes whitespace and nothing else.
Is there a size limit?: No.