Most companies use coding challenges to asses candidate's technical abilities. In most cases, they put this assessment prior than any other interview or discussion. Many developers have gone through processes of a few challenges like this.
The whole process is taking place in a special website. Such websites may provide an online IDE with some features like intellisense or just provide a way to upload the final source code. Some of them have automated tests which indicate if your code works expectedly or not.
Coding websites are not dedicated to interview questions only. Many of them have an assessment path which is like a learning course or like a tournament. People can register for, and follow each step. By this, they increase both their coding skills and their rank. It's common for companies to hire from high rank members of the website.
Not all coding challenges are problem-solving only. Some challenges are based on technologies rather than algorithms. For instance, they may be based on React.js, Laravel or Django.
Do you know any other coding website?