Unlike REST API where we consume different endpoints depending on the resource we need, in GraphQL you only need one for everything.
As of August 2018, ApolloServer has migrated from 1.0 to 2.0 with significant changes to the patterns and code (see here and here).
The tutorial divides the language types in a schema directory, which is great for code maintainability, but it also means you have to include the tag per file (_input.js, _mutation.js, _query.js, _type.js, graphql/index.js).
Moreover, graphiqlExpress and graphiqlExpress are replaced by ApolloServer, which you can wrap as a middleware to app.
GraphiQLExpress and GraphQLExpress have been replaced by ApolloServer with the additional typeDefs and resolvers as parameters.
Take note of the schema variable, as it is imported in graphql/index.js.
If you followed the instructions properly, the GraphQL Playground should show up in your specified endpoint.