Which admission am I applying for if Im applying right from high school senior year?

I’m an international student. You have all these fields of studies with a degree program - AB, MA, PhD and I don’t understand it. If you are going for the AB you’re applying for the undergraduate admission (that means applying right from high school senior year, correct?). But if there’s a field/subject that has only MA program, does that mean that you have to finish the AB first and THEN you can go for the MA? Thanks!!!

Do you mean BA? First you get a BA or BSc, then you get a Masters,then you get a PhD. I actually think this is pretty universal, not just in the USA.

Although there are a few extremely competitive BA(or BS)/MD combined programs, and maybe something similar for grad schools, but the vast majority apply for undergrad alone first.