declare type with React.useImperativeHandle()

I recommend you use type definitions more explicitly

For example, with React DT, you can define ref exotic component with ForwardRefRenderFunction instead of FC.

type CountdownProps = {}
    
type CountdownHandle = {
  start: () => void,
}
    
const Countdown: React.ForwardRefRenderFunction<CountdownHandle, CountdownProps> = (
  props,
  forwardedRef,
) => {
  React.useImperativeHandle(forwardedRef, ()=>({
    start() {
      alert('Start');
    }
  });

  return <div>Countdown</div>;
}

export default React.forwardRef(Countdown);

and then use React utility ElementRef, TypeScript can infer exact ref type of your component

const App: React.FC = () => {
  // this will be inferred as `CountdownHandle`
  type CountdownHandle = React.ElementRef<typeof Countdown>;

  const ref = React.useRef<CountdownHandle>(null); // assign null makes it compatible with elements.

  return (
    <Countdown ref={ref} />
  );
};