The questions for the military competancy test come from the same question bank as the commercial exam.
When you become a rated aviator, you can take the results from the equivalency test to the FSDO and get a commercial and instrument ticket for the category and class you are rated to fly. Once you have the commercial ticket (or if you had one before going into the military), but then go fly a different platform, you can get the new rating by showing your commercial license and the proof of the new type. I.e. you are an army aviator with the commercial helo ticket, then transition C-12s, you can get the MEL added by simply taking your paperwork to the FSDO. No additional test needed.
On your initial, you will get tickets for all platforms you have flown in the past year. A Navy helo pilot right after winging will get the helo and SEL (for the T-34). A Navy FW pilot will get the MEL and SEL (SEL for the T-34 and T-45, MEL/CL thrust restricted for the T-2, MEL for the F-14, S-3, T-44, etc.)
One caution: You must have flown 10 hours in the A/C category in the past year in order for the times to apply. A certain aviator (me) was lazy and did not do the equivalency stuff for 4 years after wings. Now I need to do the full check ride in order to get the SEL added. $300 for a check ride (plus the plane time) just because I didn't have the foresight to knock it out right after I got winged is an expensive lesson!
P.S. I know of people who fly C-12s, etc who have worked an ATP by taking a FSDO examiner for a check flight. Don't know how they did it, but nothing like doing the check ride in your own airplane.