Why does { [K in keyof M]: { type: K; payload: M[K] } }[keyof M] preserve an event name-p…

Why does { [K in keyof M]: { type: K; payload: M[K] } }[keyof M] preserve an event name-payload relationship?

1. It builds one complete object per key before collecting those objects into a union.
2. Indexed access validates every payload at runtime.
3. Mapped types turn the result into a nominal type.
4. keyof keeps only the first key in M.