GraphQL with Mongoose and MongoDB tips on deeply nested objects in arrays

Question

I can't find any answers or tips on how to work with deeply nested data with GraphQL and Mongoose. I would've thought this is a fairly common thing to do, but the docs are also pretty vague when it comes to stuff like this.

Here's how my data should look like. It is basically and a collection of invoices. Each invoice data for that invoice like customer name, invoice number, etc. It also contains an array of sections. Each section has data about that section like the type of products, color of products, etc. Each section itself contains an array of invoiceLines, and they all contain a product that takes properties from the section it is contained in, and also has it's own data.

Pseudo code:

{
    "invoices": [
        {
            "_id": "123456",
            "invoiceNumber": "i2022-123",
            "customer": "John Doe",
            "date": "2022-11-02",
            "sections": [
                {
                    "type": "phones",
                    "color": "red",
                    "invoiceLines": [
                        {
                            "product": "iPhone",
                            "year": "2022"
                        },
                        {
                            "product": "Samsung",
                            "year": "2021"
                        }
                    ]
                },
                {
                    "type": "tablets",
                    "color": "black",
                    "invoiceLines": [
                        {
                            "product": "iPad",
                            "year": "2022"
                        },
                        {
                            "product": "Samsung tablet",
                            "year": "2021"
                        }
                    ]
                }
            ]
        },
        {
            "Another": "Invoice"
        }
    ]
}

My GraphQl queries look like so:

const query = new GraphQLObjectType({
    name: 'RootQueryType',
    fields: {
        getInvoices: {
            type: new GraphQLList(ProjectType),
            resolve(parent, args) {
                return Project.find();
            }
        },
        getInvoice: {
            type: ProjectType,
            args: { id: { type: GraphQLID } },
            resolve(parent, args) {
                return Project.findById(args.id);
            }
        }
    }
});

Question #1: How would I query a specific section or an invoice line? they all have MongoDB IDs, but for some reason I can't use that to query them.

const { Project } = require('../../models/Project');
const { SectionType, SectionInputType } = require('../TypeDefs/SectionType');
const ProjectType = require('../TypeDefs/ProjectType');

const mutation = new GraphQLObjectType({
    name: 'Mutation',
    fields: {
        // Add a Project
        addProject: {
            type: ProjectType,
            args: {
                date: { type: GraphQLString },
                invoiceNumber: { type: GraphQLNonNull(GraphQLString) },
                customer: { type: GraphQLNonNull(GraphQLString) },
            },
            resolve(parent, args) {
                const project = new Project({
                    date: args.date,
                    invoiceNumber: args.invoiceNumber,
                    customer: args.customer,
                    sections: [],
                })
        
                return project.save()
            }
        },

        // Add a Section
        addSection: {
            type: SectionType,
            args: {
                // MongoDB ID for the project the section belongs to
                id: { type: GraphQLID },
                section: { type: SectionInputType }
            },
            async resolve(parent, args) {
                const newSection = args.section;
                
                return await Project.updateOne({ _id: args.id }, {
                    $push: { sections: newSection }
                })
            }
        },
    }
});

I'm using $push to add a section to the invoice.sections and that works perfectly because I can get a hold of the invoice by the MongoDB ID.

Question #2: In that case how would I be able to add invoice lines to these sections that I add with this method, since I'm not able to get a hold of the sections by their respective _id.

I guess my main issue is that I'm not able to get a hold of nested MongoDB IDs.

Any help would be appreciated, or any pointers to good resources for GraphQL and Mongoose.

P.S. Yes, I'm new to GraphQL, but I like the concept of it so I wanted to explore it.

I've tried resources from YouTube and from graphQL docs, but pretty much everything is vague when it comes to a problem like this. I would think the deeply nested data like this is a common occurrence, but I can't find proper resources