Short answer: attending an international school my whole life.
Here’s my journey to becoming a Japanese and English bilingual, without spending any time studying abroad.
My first exposure to English was at a pre pre-school that I don’t even remember attending. Along with other babies – at this point we weren’t even able to speak – and our mothers or fathers, I sat on my mother’s lap bobbing up and down to the beat of nursery rhymes in English. We would listen to the ABC songs on repeat while some ran around the room and others standing with their knees casually hopping to the beat of the song.