The interview process is outlined on the Automattic website, the first stage is an interview with a developer. The next step is a code test and the last stage is a trial.
My initial interview had very little to do with JavaScript and was about general programming aspects like testing and problem-solving. I was asked a system design question, which caught me off guard, but otherwise, it was straightforward.
I did not make it past the code test stage, but some notes about it:
1. You're invited to a Github repo with a 'broken plugin'. You're given 6 hours to complete a large variety of tasks, both frontend and backend.
2. They say to 'not spend more than 6 hours' but in reality, if you want to pass the test and impress them, you'll likely need to spend 12+ hours on the test. This is all unpaid work.
3. Vague feedback is provided if you fail the test. I mostly got vaguely defined nitpicks about my submission - 'error handling could be better' etc.
I would say, clarify with them what you're expected to do and how far you're expected to go early on, that's something I didn't do and I think that made a difference.
I will say that communication was great and everything happened in a timely fashion. I would just set expectations rather high if you're applying. They're looking for a seasoned developer that's willing to spend the time to impress them.
