alright dude since you're not giving up on your quest for an answer.. i'll help you out. you'll learn this when the time will be appropriate for you to know this but since you're so eager. here's the answer..
some specialties like Anestheisa, Derm, Rads, Neurology, PM&R, maybe ophthalmo too?, maybe a few others that i am missing as well. but basically those residencies don't start until your pgy2 year, because they're pretty specialized the accrediting bodies thought it would be a good idea to teach these guys some medicine before they forget everything they learned in med school and focus on their specialty. So if you go into one of those specialties you'll do your first year pgy1 or internship, in either medicine, surgery, or transitional year, some residencies have strict guidelines as to what you can do for pgy1 year i.e. 6 months of internal medicine for neuro or, certain amount of months of ER and ICU for Anesthesia, some residencies don't care what you do as long as you do your internship in something more general. So that's why you'll see more people match into the pgy2 residency because technically that's when radiology starts. but some residency programs decided to partner up with the hospitals IM department and have their residents do their frist year in that department, or rotate through a few departments in that hospital, this is called a categorical residency, and it's created because it's more convenient for the resident, you only have to apply to one program and you don't have to move around so much and you can do less interviews, however most of the residencies are still advanced residencies i.e. start at the pgy2 level because that's how they were historically. So even though some might match into a radiology residency as a pgy1 that's because that hospital offers a categorical residency those people will still not start their radiology residency until pgy2.
you apply to both the advanced residency and internship at the same time. you just don't start your main residency until a year later. and you can apply to a categorical residency too, but usually those are harder to get into because they're more convenient and there are less of them. Some specialties are moving more towards the categorical residencies and phasing out the advanced ones but this will take sometime. Also some hospitals might not offer a categorical residency and if you really want to go there for your residency you'll have to apply for separate internships either in surgery, IM or transitional year the surgery and im are called preliminary years. Also some hospitals might have both categorical residency spots and advanced residency spots for the same specialty and you can apply for both to increase your chance of matching at that program.
lastly if someone drops out you can transfer into residencies of course it's possible but just like transferring from a caribbean medical school to a US med school is possible it's very unlikely, well it actually might be more likely to transfer into a residency, cause people change their minds, get pregnant, or whatever.. Some just realize they hate sitting in a dark room all day and not talking to anyone.
i hope this answered your questions. if you don't understand some of the stuff i said don't worry you'll figure all this out once the time comes. right now you should focus on getting into med school, preferably a US one.
good luck.
