They companies have their accounts with Hackerrank and have some bundles for different set of questions (mostly provided by hackerrank itself[1] and could be self created, but mostly they use the questions provided by the hackerrank) - Python, Ruby, Algorithmic, Testing level- and based on the position they'll invite a candidate to try out the questions (via email).
The hackerrank site itself gives a minimal text editor of the language of your choice along with an execution environment. Of the 5 interviews I attended 1 company asked to try the questions while applying itself, 2 companies have sent the invite after a phone level screening and 2 companies invited to their office to attend the same in front of them.
But, some of the questions are more obvious (FizzBuzz, ChessBoard) that often come up in the generic hackerrank for developers section itself..
[[1] - to know the interviewer part I signed up as a Employer which has 14 day trial period and got all their interview questions in stock]
