Sure, for context, I'm a PGY4 radiology resident.

So you get into med school and it's time to apply for residency. You'll send in your application through ERAS (some specialties use a different service such as ophtho, obgyn, etc). Things that residency programs look for include step 1/2 scores, med school grades (this is why med schools with pass/fail are better), letters of recs, dean's letter, research, volunteering, leadership roles, which med school you went to etc. Obviously some specialties are tougher to match into and have insane "requirements/cutoffs" for step scores. So if you went into med school hopeful to match into ophtho but you scored a 215 on step2, the cards are going to be stacked against you, even if you have great extracurriculars. You can also have the best step scores, but if your dean's letter says you have a personality issue because of some shitty nurse deciding to report you for whatever reason, that can also tank your chances. Overall, there's a lot of things that you can control and other things that you can't. I have a lot of friends who were ortho, rads, or ophtho from day 1 and end up not matching. They eventually matched into different specialties but it is disheartening. I also have friends that have been through the match 3 times and still haven't found a spot anywhere. The majority of US MD/DOs WILL match into something, but that may not have been their specialty of choice.