graphql-tools difference between mergeSchemas and makeExecutableSchema

So the reason I am asking this question is because I can get both of these to return a working result with just replacing one or the other. So which is the right one to use and why?

What are their purposes in regards to schemas?

import { mergeSchemas } from 'graphql-tools'

import bookSchema from './book/schema/book.gql'
import bookResolver from './book/resolvers/book'

export const schema = mergeSchemas({
    schemas: [bookSchema],
    resolvers: [bookResolver]
})

import { makeExecutableSchema } from 'graphql-tools'

import bookSchema from './book/schema/book.gql'
import bookResolver from './book/resolvers/book'

export const schema = makeExecutableSchema({
    typeDefs: [bookSchema],
    resolvers: [bookResolver]
})

Both of these examples work and return the desired outcome. I believe the correct one to use here is the makeExecutableSchema but not sure why the first one would work?

EDIT Just incase it would be nice to have the types/resolvers:

typeDefs

type Query {
    book(id: String!): Book
    bookList: [Book]
}

type Book {
    id: String
    name: String
    genre: String
}

Resolvers

export default {
    Query: {
        book: () => {
            return {
                id: `1`,
                name: `name`,
                genre: `scary`
            }
        },
        bookList: () => {
            return [
                { id: `1`, name: `name`, genre: `scary` },
                { id: `2`, name: `name`, genre: `scary` }
            ]
        }
    }
}

Query Ran

query {
  bookList{
    id
    name
    genre
  }
}

Result

{
  "data": {
    "bookList": [
      {
        "id": "1",
        "name": "name",
        "genre": "scary"
      },
      {
        "id": "2",
        "name": "name",
        "genre": "scary"
      }
    ]
  }
}

Solution

mergeSchemas is primarily intended to be used for schema stitching, not combing code for a single schema you've chosen to split up for organizational purposes.

Schema stitching is most commonly done when you have multiple microservices that each expose a GraphQL endpoint. You can extract schemas from each endpoint and then use mergeSchemas to create a single GraphQL service that delegates queries to each microservice as appropriate. Technically, schema stitching could also be used to extend some existing API or to create multiple services from a base schema, although I imagine those use cases are less common.

If you are architecting a single, contained GraphQL service you should stick with makeExecutableSchema. makeExecutableSchema is what actually lets you use Schema Definition Language to generate your schema. mergeSchemas is a relatively new API and has a number of open issues, especially with regards to how directives are handled. If you don't need the functionality provided by mergeSchemas -- namely, you're not actually merging separate schemas, don't use it.