AMGs need to pass step 1 within the first 2 years while in school, otherwise, will be kicked out; with only step 1, they can apply for residency, and may be accepted, however, he/she must finalize step2 prior to the start of residency,because, without these 2 steps, state medical board will not issue an institutional license. also, all residents are required to pass step 3 by the end of 1st year of residency, otherwise, he/she cannot continue.
The difference regarding matching into residency for AMG vs FMG is that FMGs must be ECFMG certified 2-3 weeks prior to the match day, AMG does not need to, but must pass step 2 prior to start.
So technically, there is no fundamental difference. But in reality, FMGs usualy have 3 steps completed prior to applying in order to get better odds, again it is not a must.