a reducer to decide how every action transforms the entire application's state

Getting Started with Redux | Redux https://redux.js.org/introduction/getting-started

Instead of mutating the state directly, you specify the mutations you want to happen with plain objects called actions. Then you write a special function called a reducer to decide how every action transforms the entire application's state.

Redux Essentials, Part 1: Redux Overview and Concepts | Redux https://redux.js.org/tutorials/essentials/part-1-overview-concepts

Redux is a pattern and library for managing and updating application state, using events called "actions". It serves as a centralized store for state that needs to be used across your entire application, with rules ensuring that the state can only be updated in a predictable fashion.