Military takes the ASVAB which is basically the SAT / IQ Test which qualifies you were certain jobs. For example Nukes in the Navy (which is considered one of the hardest programs for obvious reasons) is 252 score for some combination of scores.
I suppose, on a sub, besides whatever admissions filtering they do for aptitude, you have to take work seriously, and also cooperate and get along with people.
When I took it in HS, I scored quite well. I’m almost certain I’m bomb it completely now. Particularly any mathematics I haven’t touched since high school would look foreign to me.