Hackers usually use phishing methods which is a type of social engineering attack. They contact people by Email or text and try to steal personal information by posing as a trustworthy organization in order to later hack into their accounts. Most of the time, people give away their username and password to these hackers without even realizing it.
Imagine getting a DM from "Instagram's Help Center" telling you that a copyright infringement has been detected on a post of yours and that your profile will be disabled unless you click on a link, log into "Instagram" and "fix the "problem". Naturally, you would get scared if you are unaware of phishing attacks, and some people will click on the link and fill out their personal information, only to find out they later got hacked. In this case, the hacker opened a fake Instagram Help Center account and set a trap, hoping people would fall into it, which in many cases they do.
If you want to see more phishing examples and learn how to protect yourself from them, we have a blog post all about it.