It was through natural interaction.
With the common knowledge of us naturally learning our first language growing up, I believe that it contributes to the idea that only children pick up learning new languages easier than grown adults. Yes it is common for children to pick up on the language but it doesn't mean that it's impossible for adults to learn. Growing up I lived overseas for some time and I remember my mom refusing to learn some of the native language to where we were because she thought that was impossible because she was "too old."