it really depends on the style, for example the last Boards of Canada disc has a bunch of guitar chords layered in with the synths.
on the chord front what you're talking about is a polyphonic synth. most modern synths support polyphony (the playing of multiple notes / sounds).
the flip side of that is an arpeggiator feature within the synth. where you play a chord but the machine plays the individual notes held down in rapid succession.
it's less about making different machines sound the same (chord synth vs. lead synth) and more about using the different features of a good synth.
not sure if it would really be possible to determine what the bass is playing from the lead melody. it would really depend on the methods the composer used, if they used counterpoint theory then it should work, but if they just improvised and found something that sounded cool then you'll have to learn it by ear.
also, welcome to the site and feel free to ask more questions there are lots of knowledgeable people here
