How do immutable update patterns and structural sharing work — what does Immer do with Proxies under the hood, and when does immutability help or hurt performance?

Imagine you have a Lego castle and you want to change one brick. Instead of breaking the whole castle, you take a photo of it, change just that one brick in the photo, and keep the old castle safe. That's what immutability does: it creates a new version with the change, but shares all the unchanged parts to save space and time.