using an element from an function inside of another function in React js

the first function is this and i want to use count inside of the second function:

const ProductListItem = ({
  item: {id, title, description, price, currency, images, category}, 
  imageSize,
  margins,
  onPress,
}) => {
    const [count, setCount]=useState(0);


    const countAddHandler = () => {
      setCount(count + 1)
    }
    const countSubtractHandler = () => {
      setCount(count - 1)
    }
    const countNumHandler= () => {
      if (count >= 0) {
        return count
      }
      else
      setCount(0)
      return count
    }

  return ( 
etc...

This is the second function:

export default function OrderCheckoutScreen({}) {
  const navigation = useNavigation();
  const dispatch = useDispatch();

  const {selected_products, loading, success, message} = useSelector(
    state => state.products.orderCheckout,
  ); 
etc...

Note that their is no big function that they are all nested in but these function are in the same file completely separated from each other.

i tried changing the first function to look like this:
export default function OrderCheckoutScreen({count})

while adding this to the first function:
OrderCheckoutScreen({count})

but i got count as undefined value.

You need to create your useState Hook outside the function/components and then pass when you call them, example:

const [count, setCount] = useState(0)
...
return (
    <>
    <Component1 count=count setCount=setCount />
    <Component2 count=count setCount=setCount />
    </>
)

By passing count and setCount as props, you can use them both inside the components and they will share the same value and the same "setCount" function.

Then, all you need to do is in the start of the component pick those values from the props object.

Component1.jsx

function Component1(props) {
    const {count, setCount} = props
    // use count and setCount normally
    return (
        whatever you want
    )
};
export default Component1;

Adapting to your case:

const ProductListItem = ({
  item: {id, title, description, price, currency, images, category}, 
  imageSize,
  margins,
  onPress,
  count,   // Add here
  setCount // Add here 
}) => {
    // const [count, setCount]=useState(0); <- Remove this line
etc...

and

export default function OrderCheckoutScreen({count, setCount}) { // Add here
  const navigation = useNavigation();
  const dispatch = useDispatch();

  const {selected_products, loading, success, message} = useSelector(
    state => state.products.orderCheckout,
  );

And finally move your useState() to the file where those components are called.

const [count, setCount] = useState(0)
...
    <ProductListItem ... count=count setCount=setCount />
    <OrderCheckoutScreen ... count=count setCount=setCount />
...