Hw3 is a physically different computer. That's easy to sort and install based on need. Tesla doesn't build cars to order anymore. They're building them in batches and then matching them based on expected delivery. If expected fsd take rate is 40% they make sure about 40% of the cars made get the new computer. My car was built after the fsd computer change, but I'm on 2.5 and I have fsd. The reason is probably that I picked an inventory car that wasn't a match and they added fsd to it.
Now when it comes to the vehicle firmware, Tesla is just building the same car from a hardware side. They could either spend the time updating the software, or ask their service centers to do it. (This is what other manufactures do), but they don't have that capacity. So they rely on the over the air update to do it.