difference between she/he is IN and she/he is AT (a country)

Question

I'm confused whether I should be saying:

1. He's in Japan.
2. He's at Japan.

They both sound alright to me but I'm wondering which one is more grammatically correct.

Answer 1

Only say "he's in Japan". We use 'at' for very local placement - he's at my house; she's at the front of the bus. We use 'in' for countries, cities, areas, etc. He's in Japan; he's in Tokyo; we're in London.

At, on or in a place (Cambridge Dictionary)