TL;DR: think of string as tuple of numbers. some are bytes, some are integers. if you want to transform those numbers into a particular encoding (e.g. UTF-8, CP-1252) then that's a different story.
EDIT: I know the article went all out about character abstraction, that why i said "some are bytes, some are integers"
That's why google starts their json with while(1); (or something like that) so any effort attempting to evaluate the json itself as javascript code will be stuck in an infinite loop.
This post does not describe the typical school in Vietnam. Unless it has dramatically changed over the last 6 years, last I recall, the most programming was taught in high school was some Hello world and how to use loops. Having said that, I was selected (via an MS Excel test) into a "gifted" team since 8th grade that was trained algorithmic programming to compete with other school districts. But that's not typical and is in no way the regular ciriculum.