PhD interview with a very general topic (no specific topic mentioned in the job portal)
Expectations in such an interview will be commensurate with your level of maturity. If you only just started your master thesis, it is indeed okay to explain how you plan to tackle the research question you are investigating. Regarding the PhD, it would be far too much to expect a full research plan for the full duration, but present the thoughts you have about the plan, and indicate an idea for a direction of a first research paper. Perhaps forecast where you might go from there, depending on the results.
The committee will know that you are in a particular phase of your masters, so you cannot be expected to already know everything about the PhD. What they would like to see is how you reason about the relevant topics, so show them how you think.