Well, understanding this stuff is basically a chapter of an introductory electronics textbook, so that's understandable.

The front panel jack connects directly to the motherboard. It's intended for headphones, so that should be the right place to connect them. It's possible the rear output will also detect that it's a headphone and connect it to the appropriate amp, but I think it's likely to have the same capabilities. There are also line-level outputs, which *may* give you more voltage than the "smart" headphone jack, but they can't drive much current so I personally I doubt you'd get better results with low-impedance IEMs. These audio chips are fairly complex, they mix multiple functions for each jack, and I don't think you can easily find a spec sheet for them, so unfortunately I can only guess.