Specifically, they use the Schulze method[1] with ranked ballots, which is far better[2] than the IRV method popularized by FairVote[3] and currently making the rounds in the U.S.

[1] https://en.wikipedia.org/wiki/Schulze_method

[2] https://electionscience.github.io/vse-sim/VSE/

[3] https://www.fairvote.org/