Don't know the point of TypeScript definition file for React Component

I know the TypeScript definition file helps us to use JavaScript version of the code with TypeScript. But I'm trying to understand the TypeScript definition file for a React component. These questions were asked by the interviewer and I don't know the answer to these questions. But I want to know. The component is written in JavaScript but also there is Typescript definition file.

Button.js:

import React from 'react';
import PropTypes from 'prop-types';
import classNames from 'classnames';
import { mapToCssModules, tagPropType } from './utils';

const propTypes = {
  active: PropTypes.bool,
  'aria-label': PropTypes.string,
  block: PropTypes.bool,
  color: PropTypes.string,
  disabled: PropTypes.bool,
  outline: PropTypes.bool,
  tag: tagPropType,
  innerRef: PropTypes.oneOfType([PropTypes.object, PropTypes.func, PropTypes.string]),
  onClick: PropTypes.func,
  size: PropTypes.string,
  children: PropTypes.node,
  className: PropTypes.string,
  cssModule: PropTypes.object,
  close: PropTypes.bool,
};

const defaultProps = {
  color: 'secondary',
  tag: 'button',
};

class Button extends React.Component {
  constructor(props) {
    super(props);

    this.onClick = this.onClick.bind(this);
  }

  onClick(e) {
    if (this.props.disabled) {
      e.preventDefault();
      return;
    }

    if (this.props.onClick) {
      this.props.onClick(e);
    }
  }

  render() {
    let {
      active,
      'aria-label': ariaLabel,
      block,
      className,
      close,
      cssModule,
      color,
      outline,
      size,
      tag: Tag,
      innerRef,
      ...attributes
    } = this.props;

    if (close && typeof attributes.children === 'undefined') {
      attributes.children = <span aria-hidden>×</span>;
    }

    const btnOutlineColor = `btn${outline ? '-outline' : ''}-${color}`;

    const classes = mapToCssModules(classNames(
      className,
      { close },
      close || 'btn',
      close || btnOutlineColor,
      size ? `btn-${size}` : false,
      block ? 'btn-block' : false,
      { active, disabled: this.props.disabled }
    ), cssModule);

    if (attributes.href && Tag === 'button') {
      Tag = 'a';
    }

    const defaultAriaLabel = close ? 'Close' : null;

    return (
      <Tag
        type={(Tag === 'button' && attributes.onClick) ? 'button' : undefined}
        {...attributes}
        className={classes}
        ref={innerRef}
        onClick={this.onClick}
        aria-label={ariaLabel || defaultAriaLabel}
      />
    );
  }
}

Button.propTypes = propTypes;
Button.defaultProps = defaultProps;

export default Button;

Button.d.ts:

import * as React from 'react';
import { CSSModule } from '../index';

export interface ButtonProps extends React.ButtonHTMLAttributes<HTMLButtonElement> {
    [key: string]: any;
    outline?: boolean;
    active?: boolean;
    block?: boolean;
    color?: string;
    disabled?: boolean;
    tag?: string | React.ReactType;
    innerRef?: React.Ref<HTMLButtonElement>;
    onClick?: React.MouseEventHandler<any>;
    size?: any;
    id?: string;
    style?: React.CSSProperties;
    cssModule?: CSSModule;
}

declare class Button<T = {[key: string]: any}> extends React.Component<ButtonProps> {}
export default Button;

In Button.d.ts file, there is an interface But,

What is the purpose of indexable-type [key: string]: any; in the interface? and

Why declare class Button<T = {[key: string]: any}> is written with Generic. Also, the JavaScript version of this component has 2 additional props.

children: PropTypes.node,
className: PropTypes.string,

But why interface doesn't contain types declaration for these 2 props?