I can only speak to Army ROTC as my DS was commissioned via AROTC. (
@GWU PNS is absolutely the expert on all things NROTC)
For Army, it depends on whether you receive a 4 year or 3 year scholarship.
If it is 4 year, once you pass DoDMERB, pass the physical fitness requirement (not sure if the ACFT is now in place or not). the program will pay for tuition, and all mandatory fees (you can choose to use the scholarship toward Room and Board but this has a cap and more limitations). Some schools (but not UC's) will throw in discounted or free room and board if you are an ROTC scholarship recipient. In addition, you receive a $1,200 cash stipend each month to go toward books and an additional stipend of $420 for other expenses.
For three year scholarships, you are on your own the first year, and so long as you are in good standing (behavior, health, fitness and grades), your scholarship will kick in for the fall of your sophomore year.
Graduating debt free is a wonderful thing. My DS just bought his first house (no help from mom/dad), on an O-2 salary and he is "allowing" his room mates to pay most of his mortgage.
For more info, here is a nice Army ROTC benefit recap from UTSA:
Benefits | Army ROTC (utsa.edu)