Good breakdown of the mental model. Two things worth adding for anyone migrating now. The fetch caching default flipped in Next 15, fetch is no longer cached by default so you have to opt into force-cache explicitly, which trips up people following older tutorials that assume it caches. And use client marks a boundary, not a whole subtree. You can still render server components inside a client component by passing them as children or props, so you rarely need to push the directive as far down the tree as people assume.