So, the first 1-2 years are for almost anyone, and then I have to apply for the 3rd and 4th year of ROTC?
"Generally" yes. But the mechanics and administration for each service branch are different.
For ArmyROTC: You must be granted a contract (or a scholarship) to advance to the MSIII status (junior year)
For NavyROTC and NROTC-Marine Option: You must be granted a contract to move to Advanced Standing for NROTC or NROTC-Marine Option.
For AF: You must pass the Air Force Officer Qualifying Test (AFOQT) to move to the Professional Officers Course (POC).
For example, my DS failed to be accepted to USNA and USMA, and did not receive an NROTC MO scholarship back in his senior year of HS (he did not apply for Army ROTC scholarship).
He was accepted to an SMC, but chose to attend a flagship university out west and entered Army ROTC as a non-scholarship cadet.
Before the end of his freshman fall semester, he earned a 3 year campus based scholarship (these are less common now). Before the end of the year, they increased his scholarship to 3.5 year and paid for his spring semester tuition. As long as he kept his grades up, stayed fit and showed good leadership, they kept him on scholarship. However, had he dropped his grades, failed to keep fit or gotten tossed in jail, he could have lost it all and in some cases had to pay back the scholarship (or serve as an enlisted).
He is now a 1LT currently an Abrams platoon leader completing his third year in the Army. The scholarship allowed him to graduate
debt-free and enabled him to buy his first house with no help from mom and dad.
Not a bad deal.