domRefs.jsx

//Class
class InputWithFocus extends React.Component {
    constructor() {
        super();
        this.inputRef = null;
    }
    render() {
        return <div>
            <input ref={inputRef => { this.inputRef = inputRef }} />
            <button onClick={() => this.inputRef.focus()}>
                Focus the input
            </button>
        </div>
    }
}

//Hooks
const InputWithFocus = (props) => {
    const inputRef = useRef();

    return <div>
        <input ref={inputRef} />
        <button onClick={() => inputRef.current.focus()}>
            Focus the input
        </button>
    </div>
}