They contact you, or at least that was how it has been historically. For some they will not do the interview at the college because they are also applying for USAFA, Our DS's ALO submitted the interview, thus he never interviewed at an AFROTC unit.
I do agree with Akrogan and try to request an interview at your top choice. There are 3 reasons for this.
1. ICSP is different than the HSSP.
~ It is dedicated to that one unit. If the college is expensive it basically acts like a Type 1. Full tuition regardless of the cost, there is no financial limit placed on it.
~ Understand if they offer you an ICSP, you are given a small window accept. If you accept you must remove your name from the HSSP program. You can't play both sides, so it can be risky if you do not have an answer from admission.
2. Some ROTC units also sit on the admission boards.
~ This can help if the school may be a reach. They can than go to bat for you on that font.
3. You can ask to also meet some of the cadets too.
~ ROTC units are unique. They all have their own personality. Meeting some of the cadets can give you a feel of what the unit is like from their perspective.
That being said, please realize that being offered the ICSP or them sitting on admissions board is not common, so do not enter thinking either are realistic options. Walk in thinking it may be an option.
Good luck