when are you Jewish? (somewhat clickbait title didnt know what else to type)

Im not Jewish myself and dont have the knowledge you would have ,so therefore i ask :))

Please dont mind my poor English :))

So I saw a post where he said he was technically Jewish because his grandfather was and therefore was wondering if he should convert to Judaism.

Is this true that you are Jewish by Jewish law if your grandparents are Jewish, does this count with other family members? father? cousin? uncle? etc.

And if its true please explain why so and how it works :))

Thank you!