How detailed should PhD proposal component of a PhD application be?
Yes indeed, I think it is expected that you would first contact the prospective advisor more informally and at minimum get an idea about the potential topics they would be interested in and that would fit with their existing research and the plans they have for the future. It's nearly impossible to guess this as an outsider.

In my case, in Germany and in CS, I had to write this for applying to the funding rather than the PhD position per se (which can be given out with almost no process by the prof). I first sat down with the prof and we talked things over and he agreed to take me on if I could acquire funding. Together with my prospective advisor and the post-doc who would be supervising me on the daily, I then wrote about 10 pages, with a detailed plan of the work I would be doing, including a gantt chart of tasks etc. Definitely not the kind of thing I would've been able to produce on my own.

I also went partway through an application process for a British university, where I had two skype interviews with the prof discussing his research before he told me to sit down and write a draft for the research plan for the formal application and send it to him for comments before submitting. (I ended up accepting another offer before I finished the draft, so I don't know what precisely they would have been looking for there.)