Reversing the battery connections is not too uncommon. If the key is off the damage is often just fusible links and fuses and the alternator. If, however, the key is on a great deal of damage can be done. Many relays, the computer, the ignitin switch, etc. If the starter was engaged, it too might have been damaged. And there is the possibility that wires, anyhwere in the harnesses, have damaged insulation but won’t short out until some future time. The McParts stores that install batteries have all paid dearly on occasion for their efforts at “free” customer service. And one of the big box stores here once sold a generic battery with the polarity marked incorrectly. They were selling and installing group 24 batteries with 24R labels and lids. It was particulary costly on a couple of cars.