In the same application season, at the same time, you apply to intern years AND radiology programs. When you make a match list, you can list your radiology program and then underneath it is a separate supplementary rank list for your intern year.
Example: you rank university of Cincinnati as your number one. And then your associated supplementary top 3 prelim programs are at or near the university of Cincinnati. Your number two is university of Missouri. You have a different supplementary rank list associated with this location in Missouri. That way you don’t have to do the prelim and the rads residency in two different places, potentially on opposite ends of the country (unless you want to for whatever reason).
IR is very competitive, even moreso than DR but that gap is closing. If you apply DR, there is an alternative pathway through ESIR where you spend your 4th year of radiology residency doing IR and don’t lose anytime. The spot for ESIR is not guaranteed, but almost always available as it’s very common to switch from IR to DR (IR is considerably less sexy as a resident). This allows you to have better flexibility with where you train, especially as a DO. Worst case scenario is that you don’t have an opening for ESIR and have to do an extra year of training to be IR, but that’s pretty rare.