Need help in understanding Expo


I am reading the documentation and I am trying several examples of React with Expo and with Gatsby.

I am following this doc:

There is a passage that says "Create a gatsby-config.js and use the plugin - touch gatsby-config.js" I already have this file and there is also some code, so I don’t need to create it, it’s an error documentation or mine?

However I add this line gatsby-plugin-react-native-web, and it seems to work properly with the yarn gatsby develop command.

At the end of the document, to add the Expo modules, i must use this command: yarn add expo, then I give the “expo start” command, the browser page opens, but i encounter the following error:

 web  Failed to compile.
Module not found: Can't resolve '../../App' in 'C:\Users\username\gatsby-site\node_modules\expo'

What can it depend on?

I have another doubt,
if I use the “yarn gatsby develop” command, I can navigate in the Gatsby versionat the url localhost:8000

If I use the expo start command, the browser opens with the Expo interface and then I have to click on “run in web browser”.

Isn’t there a command that combines the two “yarn gatsby develop and expo start” or do I have to use them both to work on web and mobile?