what part goes bad when they go bad? Do the cups get ground down, or is it the gear or bearings???

are the axles an easy part to locate?

I replaced both inner and outer boot on my passenger side after I noticed a tear, but no noise and will be replacing my driver side inner and outer boot due to dry rot that will become a tear. I am hoping that will be good for awhile, but its always nice to know about my backup plan.