How do American immigrants learn English?

You can't talk nonsense about this. According to Jim Cummins' research in Canada, you already have a basic knowledge of English, and you will muddle along every day for 2/3 years. When you enter the major, you will understand through reading, although it is still not up to the level of local students, and it will take 5/7 years. You can find it online, and you dare not talk nonsense.