explain to me how pilot selection works?
A quick search on google will give you a detailed answer but in short you apply for Rated Board in the spring semester two years before you graduate. Your GPA, PFA, FT report, AFOQT, and PCSM scores are accounted for that selection. You will be notified if you are selected about a month after you submit your package. If selected you go to Wright-Patt for a thorough medical exam to see if you are fit as a pilot. Commission and you go to pilot school.
can you be denied a rated slot for field training but be accepted into a non-rated slot?
Doesn't happen that way. EA boards are separate in that you will compete only for Rated non-tech/tech if you chose that when you get nominated. Now there is a rolling EA board that continues to select people that didn't get selected at first but you are still considered a Rated EA. After FT and you go up to the Rated Board but didn't get a rated slot you will proceed to be a non-rated cadet until a rated slot opens up.
Also, can somebody explain the types of EA's
Rated Tech, Rated Non-Tech, Nonrated Tech, Nonrated Non-tech, Nursing. Rated are for cadets that are medically qualified and desire to be Pilots, CSOs, ABMs, or RPA pilots. Tech majors are technical majors mostly in the STEM community. Anything else is considered non-tech.