Where should react-router be set?

recently, in learning react, I would like to ask an experienced friend, in general, is the router of react set in app.js or index.js?

Oct.27,2021

// App.js
class App extends PureComponent {
    render() {
        return (
            <HashRouter>
                <Provider value={{ data }}>
                    <Route exact path="/" component={CategoryList} />
                    <Route path="/question/:id" component={QuestionDetail} />
                </Provider>
            </HashRouter>
        );
    }
}

I wrote
app.js and index.js in the App.js file. Router is just a component.

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