What determines the component of the army (Active, Reserves, ARNG) you commission in after completing ROTC?

Needs of the Army, performance which leads to placement on the order of merit list, and preference

https://www.rallypoint.com/command-post/how-do-i-choose-an-army-branch-a-framework

You need to Google ROTC accessions and OML. There is a point based system that is used by cadet command to determine your component and branch assignment.

Once you've done that, you should spend time here on RP discussing the finer points, like how to balance school and ROTC work-loads, different training opportunities available, tips on improving OML score, etc.

RP could also offer you down-to-earth wisdom in the road to comissioning. For example, when I was a cadet I had a bit of a speech I gave other cadets trying to convince them to choose active duty out of college. And I always spent a little time with those cadets who were prior service. The biggest failures I saw in the program we're not the ordinary college kids, but the prior enlisted who stubbornly and arrogantly held onto their idea of superiority and closed their minds to learning a different form of leadership. I made that mistake myself.

Here's how things went for us:

1. OML determines the cut off for active duty vs Reserve/National Guard component.
2. You had a selection of 3 preferred branches and duty stations
3. You received your assignment based upon your position in the OML and the needs of the Army.
4. You got notified by your cadre about what the determination was.