Say you added a new collection type named "likes" and Strapi automatically creates graphQL API for you, you can customize the API by doing this:

• Update api/likes/config/schema.graphql.js
  module.exports = {
query: ['findtest: Likes'],
mutation: ['updatetest: Likes'],
resolver: {
Query: {
findMyLikes: {
description: 'Return a list of all my likes',
resolverOf: 'application::likes.likes.find',
resolver: 'application::likes.likes.findMyLikes'
}
},
Mutation: {
deleteMyLikes: {
description: 'delete all my likes',
resolverOf: 'application::likes.likes.update',
resolver: 'application::likes.likes.deleteMyLikes'
},
},
}
};

• Update api/likes/controllers/likes.js
  `
  module.exports = {
  findTest: async (ctx) => {
  let t = await strapi.services.likes.findOne({id: 5});
  return t;
  },

updateTest: async (ctx) => {
// const {id, categoryID} = ctx.request.body;
return {
id: 4
}
}
};
`

That is it!