as far as how they get just the vocals from a sample, there are several ways. On a professional level itmay be possible to get the original vocal track from the master recording session. Onbviously that is more likely with newer songs but not totally impossible for all older songs. Another way is the most obvious, to take it from the acapella of the original if one is available. Most older songs did not release acapella versions, BUT every now and then you stumble across one (usually in the form of an impromptu live session). The third way i will mention is via filters. This takes some practice and rarely do you ever get a perfect ly clean sample but i have obtained some decent acapellas this way. often whatever remnants of other sounds remain blend into whatever music you place it on top of. Hard to explain in detail, but the gist of it is that you estimate what frequencies the voice lie at and filter out everything else. It';s pretty hit or miss but an option.
