if you talk about music production, it is done in many ways. Usually instruments and vocals are recorded separately and then the process in which all layers are joined together in a studio which determines the level of each layer is called production. this is the process where the producer decides whether the guitar should sound above or below the vocals.
as for remixes, as the name suggests, the original track is used and multiple other layers are added or 'mixed' with it, hence the name 'remix'. the additional layers can be different, synthesizer based, software based, etc. some effects can also be added for example the turn table which the DJs use. usually softwares are incorporated but it doesn't mean that its a piece of cake, creative juices must be flowing !