I passed three interviews: one with the PM (if I understood correctly) and two further technical interviews, each with a different single developer. All in all, this, as I understood it, comprised the full interview process.

After waiting several days for their final answer, I decided to reach out myself for an update. I received a response stating they are looking for 'even more senior' candidates than me.

This response was quite strange because, in my opinion, I passed all interviews successfully. When I perform weakly at solving a problem, I recognize it. However, in these interviews, I genuinely don't understand what they disliked.

Solving some not very hard algorithmic problems, and revealing problems in C++ code.