How do React Server Components change the client/server boundary, data-fetching architecture, and bundle size compared to traditional SSR and client-side rendering?

Imagine you have a toy that needs instructions. Normally, you get all the instructions and the toy, and then you build it. With React Server Components, your friend reads some instructions for you before sending the toy, so you get a partly built toy with fewer pieces to put together. That makes it faster and easier for you to play!