For example, how were dents in plate armor repaired? What about holes? Was it cheap to have armor repaired or was it more common to throw damaged armor out and have it be replaced by a new one? Did they just hammer most of the damage out, or were there any complicated processes that they used?
I cant speak for most styles of armor, but this is one of the biggest reasons why chainmaille remained so popular for so long. Though the construction process is extremely tedious, most damages to chainmail would only result in the loss of a handful of rings. From personal experience, Ive shot patches of chainmail with bullets and arrows, and this usually loses one ring, maybe two. I never tested it against a axe/sword/spear, but on other videos Ive seen it has been <5 rings.
For a soldier in the field, they would be able to put in a jump ring themselves as a temporary patch. This is just a loop of metal, cut on one end so that you can twist it open and close. Modern day we use two sets of pliers, I know you can do it with chopsticks. Either way, temporary repairs are amazingly easy.
Permanent repairs you have to go to the blacksmith so that they can rivet they rings shut. These rivets are important, as the rivet increases the rings strength by about 4-10x. But for an individual ring your still only looking at about 5 min work to install the rivet and heat treat. And for those larger sections with 5~ rings I’d still only imagine 15 min as you get economies of scale.
There are a few other forms of armor that have similar advantages. Scalemaille actually skips half the rivets, while adding strength because it can now deflect strikes rather than let attacks in to pop a ring. Brigandine likewise can have internal plates swapped out. To my knowledge, though chainmaille was one of the longest lived and I believe it was in large part due to the ease of repair.