Good question. Here's my understanding of it, one of the doctors can correct me if it's rather more different in practice
You're right, during your time at med school you won't be able to see everything. However, for you as a budding FY1 this doesn't really make a difference. Perhaps it would make more sense if I used an example.
At my med school during our first clinical year we do a surgery block. In this block we are allocated to two surgical wards (out of general, cardiothoracic, neurosurgery, vascular surgery, and urology). So some students may end up seeing 8 weeks of cardiothoracic surgery and colorectal surgery, but no urology. Does this matter in practice? Not really.
When you qualify and are allocated to a surgical ward as an FY1, it doesn't really matter what kind of surgery the patients are in for. Your job will still be roughly the same wherever you are i.e. you'll need to sort out the bloods, chase results, do the TTOs, follow the consultant on ward rounds etc. It doesn't really make that much of a difference if you're on a cardiothoracics ward or a colorectal ward. Obviously some procedures will differ from ward to ward and specialty to specialty, but you'll have (theoretically) gained a good general grounding in med school and seen enough to be a competent surgical FY1 for your four month allocation, even if you haven't experienced every specialty. And to pass finals you'll have had to read around the specialties you haven't seen anyway, so it's not like you'll be completely unfamiliar with urology or whatever.
Now, if you're particularly keen on being a urologist during your time at med school, you could always approach one of the urologists and ask if you could do a swap with one of the other students so you can sit in on their clinics, theatre lists etc for one week...that's cool and all, but it's not like you'll be rejected for urology specialty training if you haven't done this!
Similarly, during FY1 and FY2 you can arrange taster weeks in other specialties so that way you can gain some experience in that particular field.
Hope that helps.