React navigation name issue

Ive inherited some code here and I'm just confused as to how the app knows where the 'NextPage' is.

I call this:

            this.props.navigation.navigate('NextPage');

With this:

import { NavigationScreenProp } from 'react-navigation';interface InitialLoadingScreenViewProps {    navigation: NavigationScreenProp<any, any>}

where is the 'NextPage' declared? so that when I create a new page I can create a navigation name for it. Ive tried using the search tool on VSCode but couldn't find it.