Agave In Tequila?

I heard agave is used in the making of tequila and mezcal.

Can I use any old edible cactus in the alcohol?

Say for instance a prickly pear ground up and extract all the juices, then add yeast and let it ferment. If it needs more sugar, add it in? How does distillation work and sugar/ yeast additions for tequila anyways?