I applied online to a PhD student position advertised on their website.
After the receipt of my application, I was asked to provide a recommendation letter. In fact, a short informal email of a previous supervisor of mine was sufficient.
Next, I was invited to visit the research group in Vienna for one-and-a-half days. While they covered my accommodation, I traveled to Vienna from a neighboring country at my own expense. Perhaps, if I had asked, they would have also assisted me with the travel.
During the visit, I gave a research talk to members of the research group as well as the professor. As is common in such talks, there was a question and discussion round afterwards. The visit allowed ample time to meet and discuss with the group members, and learn what they are working on. I also had a one-on-one meeting with the professor, where he explained my prospective research topic in more detail and I was free to ask questions. I never had the feeling I was being grilled. The entire group was very amiable and it was a pleasant experience. For example, they joined me for dinner.
I received an offer by email shortly after the interview.