What's the difference between settlement and immigration? Is there any institution in Changzhou that handles the settlement of immigrants?

Settle down, as the name implies, is that you once lived in the past.

Immigration means that you get an overseas status and are eligible for permanent residence, but you don't have to live there.

Many people emigrate for various reasons, but still live in China.

There are many immigration agencies now, and you can see a bunch of advertisements by just searching. Choose a few and compare them yourself.