The question
"A friend says he 'owns Reliance'. He owns 12 shares worth about ₹15,000. Is he joking, or is it true?"
It is true. Not "sort of", not "like owning" — he legally owns a piece of one of the largest companies on Earth. This chapter explains what that piece is, why any company would sell pieces of itself, and why the price of that piece moves every single day.