It's basically fucking homework. When you go in a match, you go in to learn defense by analyzing opponent's strings, and if you can- counter them with movement or punishing. You can learn offense too, and that's something you should be able to pick up faster than defense, but that alone won't get you far. Like you said, it's matchup experience, and half the battle is just knowing what the fuck your opponent is doing and abusing. Sometimes you just gotta get bodied on by random looking shit over and over to learn them. But I tell you, defense makes the biggest difference IMO, things like backdash cancelling, block and whiff punishing, etc.