> How can you store program state on disk like that? You'd need serialization and deserialization.

You need serialization and deserialization when interacting with a DB, too. You're trusting a library to do most of that for you. This is the difference between a junior engineer and a senior engineer. As a junior engineer, you don't trust your own judgement and use a library. As a senior engineer, you don't trust the library writer's judgement, so you write it yourself.