The App_Config\Sitecore\Services.GraphQL\Sitecore.Services.GraphQL.Content.config is from the JSS package, no need the change there somethings.
It is about your GraphQL endpoint, In your end point your configer what you need.
Or you can use a predefined config like the. /sitecore/api/GraphQL/defaults/content/schemaProviders/systemContent
In your react app ...
It is feasible and you just need to expose the API. So, on your react component, you will have a button whereby you will bind it to a function.
In that particular function, you can use Axio to call your endpoint (contact/submit).
Then, it is pretty much straight forward. You need to implement the API and register it in Sitecore so that when the url mysite....
Make sure your top level placeholders have been added to rootPlaceholders setting in package.json file:
Unless the root level placeholders are set, the placeholders and their components do not render in ...