You wrote that you interviewed in central Europe without giving a country name. For matters such as yours, the country makes a substantial difference.
Consider this answer to be one for Germany, which is somewhat centrally located in Europe.
The most common way to perform PhD studies in that country is to get a "scientific employee" position with the university while doing your PhD. The positions are allocated to professors, not to schools. Hence, you didn't actually interview with the school, but with the professor.
This explains why there was only one person. While the style to first give a scientific talk to all members of a research group, followed by a (confidential) interview is somewhat common (for a good reason), going straight to the one-on-one interviews is also not uncommon. Sometimes there are logistical reasons for skipping the talk. Oh, and if the research group only consists of the professor so far, there is nobody else to give a talk to. Whether members of other research groups are invited to such talks depends on local customs.
I would personally not consider the odd knowledge testing a red flag. The reason is that school and university only do a very formal check if you bring in the right degree to start PhD studies with you. It is possible to pass this check but still come from a university where the teaching quality is so bad that you have no chance of successfully completing a PhD program. So it's up to the professor to check this, but this is insanely difficult in a short interview. Doing multiple rounds of interviews, taking hours overall (such as in many tech companies) is unusual, and would probably not be accepted by every candidate. To test the broad understanding of the field of research, the professor may have prepared some questions that can only be answered with this broad knowledge and cannot be answered by knowing some factoids. Secondly, many candidates with a less formal education do not use technical terms and expressions correctly and interpreting whether they mean to say the right answer or just combine some concepts to a sentence is sometimes very difficult. Some questions may sound weird, but at the same time have a good answer that is hard to guess so that traces of the correct solution can already count as a correct answer.
What you describe afterwards is mostly a lack of proper organization. This can be a red flag if it occurs multiple times (such as in your case).
For the students starting in 2020/2021, you unfortunately don't know what happened. They may have let go (but that is unlikely if they were working there for >6 months). Some of them may have decided late that the PhD is not the right path for them. Dropout rates of ~50% are common in Germany, even at the PhD level. The professor may have had back luck with the students, may have hired the wrong ones, or may not be a good advisor. Unfortunately, that's hard to tell. It's also possible that these students only were supposed to start back then, but couldn't obtain a visa, and finally went elsewhere - many embassies closed due to the Corona virus at that time. Normally, PhD students are not added to web pages before they arrive, but this is not uniform.
