I just had a conversation with a young man, who did pretty poorly in school, failed high school, etc. However, he spoke English really well. A bit of an accent, but very fluid.
I asked if he had lived in England or the US or anything, but no. He said he learned his English from...playing video games! These are the types of games where you're constantly talk with your fellow players with a headset.
In case any English speakers are wanting to learn other languages - well, that doesn't really work, since apparently the "lingua franca" of all the video games is English.
The other surprising language learning methods I've heard of are...binge watching TV shows in English. When traveling through France a few years back, two separate times I talked to people who spoke English really well, very fluently.
When that happens, I used to assume that they lived in an English speaking country for quite some time. But both of these guys said no, they just...binged watched TV. For the one it was the show Peaky Binders, and for the other, it was The Big Bang Theory.