An arc welder of any type is at heart just a power supply. The Tig torch takes that power and puts the arc out through a small diameter tungsten electrode that is not consumed in the process, the shielding gas prevents the tungsten from burning up as well as protecting the weld puddle from the oxy in the air. The small diameter means less power is needed to make a small puddle and the concentrated heat makes the heat affected zone smaller.
When arc welding with a consumable electrode, you replace the Tig torch with a standard electrode holder and leave the gas turned off. Now it operates just like a normal DC arc welder, but with only 90 amps of power it is suitable only for smaller/thinner work as you won't get much penetration.
I lucked onto a ThermalArc welding power supply at work, it does TIG, Stick, and MIG. There is a switch for each mode, but that is mainly because it has several current modification controls and some are not desireable for a particular type of welding. It came with a Miller wire feeder with a feeder gun on it for AL wire. It works for all 3 types of welding, I just need to get more practice in, especially on the AL MIG.