That is not a stupid question at all- it was one of my big worries about DMU, and after being at DMU for a year it makes a lot more more sense. This might be a long reply, but I'm going to try and explain it fully. You've essentially got it right that the difference is that you're at one health system in a year-long, while non-year longs aren't at the same hospital.
So there’s two ways that you can do rotations: year-long, and non-year long. Of the 220 students in this year's 2nd year class, roughly 160-170 ended up in year long sites, but the # of year-long sites will be increasing in the next few years. Here's what the differences are:
1) Year Long: These rotations are entirely set up by the school and selected using a lottery system in spring of 2nd year. The rotation sites typically are in the Midwest with a few being further away. I would estimate that around 60-70 students get placed in year-long sites in the Des Moines area, with probably another 50 in Iowa communities (other than DSM), and another 50 in locations elsewhere (Indiana, Texas, Michigan, Wisconsin, Minnesota, etc). You can find the past year's sites on the DMU website, although obviously they are subject to change. Year-long sites provide all of the necessary rotations for 3rd year in one physical location. This is probably what you traditionally think of when you think about rotations.
2) Non Year Long: The other option is to have non-year long rotations. The big difference is that in non-year long, you can be in various hospitals and clinics throughout the year. Typically 8 or 9 of your 10 rotations are in the Central Iowa area (within 30 minutes of DMU) and 1 is an "out" rotation, which is still in the midwest, but may be a little bit further away. For example, you may have family medicine at Broadlawns in Des Moines, pediatrics at Unitypoint-Blank Children's, psych at MercyOne-Des Moines, etc, and then maybe OB/Gyn in Central Nebraska (or at some other site around the Midwest). The cool part of this is if you know a physician that you want to set up a rotation with, DMU will work with you to try and coordinate that. But if you don't know any physicians that you want to rotate with, DMU will set up everything for you! You can set up either a few rotations with physicians you know, or you can even set up your entire 3rd year, if you're able to do that. DMU is helpful in coordinating everything, and it's actually a lot less stressful than it seems. Some students actually prefer to have non-year long rotations because it allows them to explore many different hospitals and settings, and makes sure they'll stay in the Des Moines area.
Just to clarify, there is absolutely no circumstance that you have to set up rotations on your own. If you want to do that, DMU will help you. But if you're non-year long, DMU will find everything for you and get them all lined up. It's a collaborative process and actually works really well.
All in all, my personal opinion is that it definitely isn’t a big thing to worry about. DMU is known to have some great rotation sites with some of the top preceptors in the country, and it’s something that they’re working hard on improving. I would bet by the time you're 3rd years, they will have a year-long rotation site for every students that wants one, and DMU will continue making sure every student has quality experiences on rotations. Hopefully that makes sense, if not- I can definitely try to answer more questions.