Pete Hunt, one of the early contributors to React, says:
You'll know when you need Flux. If you aren't sure if you need it, you don't need it.
Similarly, Dan Abramov, one of the creators of Redux, says:
I would like to amend this: don't use Redux until you have problems with vanilla React.
In general, use Redux when you have reasonable amounts of data changing over time, you need a single source of truth, and you find that approaches like keeping everything in a top-level React component's state are no longer sufficient.
However, it's also important to understand that using Redux comes with tradeoffs. It's not designed to be the shortest or fastest way to write code. It's intended to help answer the question "When did a certain slice of state change, and where did the data come from?", with predictable behavior. It does so by asking you to follow specific constraints in your application: store your application's state as plain data, describe changes as plain objects, and handle those changes with pure functions that apply updates immutably. This is often the source of complaints about "boilerplate". These constraints require effort on the part of a developer, but also open up a number of additional possibilities (such as store persistence and synchronization).
If you're just learning React, you should probably focus on thinking in React first, then look at Redux once you better understand React and how Redux might fit into your application.
In the end, Redux is just a tool. It's a great tool, and there's some great reasons to use it, but there's also reasons you might not want to use it. Make informed decisions about your tools, and understand the tradeoffs involved in each decision.
Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code. That said, it is most useful when combined with a declarative view implementation that can infer the UI updates from the state changes, such as React or one of the similar libraries available.
<script> tag. As the relevant pull request says:
The new Counter Vanilla example is aimed to dispel the myth that Redux requires Webpack, React, hot reloading, sagas, action creators, constants, Babel, npm, CSS modules, decorators, fluent Latin, an Egghead subscription, a PhD, or an Exceeds Expectations O.W.L. level.
Nope, it's just HTML, some artisanal
<script>tags, and plain old DOM manipulation. Enjoy!