I agree with Jcleppe use the search tools here ad go to their actual ROTC website, than come back if you need some more clarification.
~ Not talking about being a walk on for ROTC....that has been answered ad nauseum, or can I do ED out of ROTC (I know for AFROTC their website explains how ED works)
I am pretty sure a few hours looking at these threads on this forum, every single question you are wondering right now will be answered well enough for you to understand the true differences between all 3 ROTC programs.
Guessing you are a junior in HS, my best tip is that you understand within a month or so many candidates will be opening up their ROTC portals. Time to research is now because the scholarship process for each branch is different.
~ NROTC boards start to meet late Aug/early Sept. and it is a queue. That means the later it takes to submit the further in the line to be boarded you will be.