Normally the production of specific alloys is a fairly specialised process and they would be bought in as ingots or other stock types from commercial manufacturer rather than being made as part of a manufacturing process. This is as much about the quality control and analysis required to produce a given alloy to a specification as the actual mechanics of melting and handling it.
When aluminium is refined from bauxite it is reduced by electrolysis, therefore aluminium refining plants are generally sited close to an economical source of electricity like a hydro-electric power station.
Once you have ingots of metallic aluminium it is relatively easy to melt and both gas fired and induction furnaces are commonly used. Aluminium is usually melted in a refractory ceramic crucible although steel crucibles can be used in some circumstances.
In order to produce a specific alloy you first need to know the composition of the metal you are starting with and then add the appropriate alloying elements in the correct proportions. Depending on the process flux may also be added to aid in separating out oxides and exclude atmospheric gasses (although aluminium is self-fluxing to a certain extent, in the sense that is quickly forms a tough oxide layer on the surface of the molten metal which is effective in excluding the atmosphere ), it may also be necessary to add material to scavenge unwanted impurities and (especially in the case of aluminium) to remove any dissolved gas.
Aluminium furnaces can be very simple indeed although, for obvious reasons the degree of sophistication will depend on the scale of production and the quality required from the finished product.
One special case is where magnesium is a significant alloying element, in this case the very high reactivity of magnesium requires special safety precautions as molten magnesium can burn uncontrollably in air and as such it is not generally wise to melt high magnesium alloys in an open furnace.