Category and subcategory API node design - node.js

Question

I am creating an ecommerce project, so there will be like a category ('men'), and subcategory of it "men's shoes", and an admin user can use addcategory in the mongodb, I found some code snippet from youtube, but the logic is so confusing, and I have no idea. can anyone kindly explain the step to step how to create this type of API?
in my category model file, I have this:
const mongoose = require("mongoose");
const categorySchema = new mongoose.Schema(
{
name: { type: String, required: true, unique: true, trim: true },
slug: { type: String, required: true, unique: true },
parentID: {
type: String,
},
},
{ timestamps: true }
);
module.exports = mongoose.model("Category", categorySchema);
in the category controller file:
const Category = require("../models/category");
const slugify = require("slugify");
function createCategories(categories, parentId = null) {
const categoryList = [];
let category;
if (parentId == null) {
categories.filter((cat) => cat.parentId == undefined);
} else {
category = categories.filter((cat) => cat.parentId === parentId);
}
for (let cate of category) {
categoryList.push({
_id: cate._id,
name: cate._name,
slug: cate.slug,
children: createCategories(categories.cate._id),
});
}
return categoryList;
}
exports.addCategory = (req, res) => {
const categoryObj = {
name: req.body.name,
slug: slugify(req.body.name),
};
if (req.body.parentId) {
categoryObj.parentId = req.body.parentId;
}
const cat = new Category(categoryObj);
cat.save((error, category) => {
if (error) return res.status(400).json({ error });
if (category) {
return res.status(201).json({ category });
}
});
};
exports.getCategories = (req, res) => {
Category.find({}).exec((error, categories) => {
if (error) console.log(error);
if (categories) {
const categoryList = createCategories(categories);
res.status(200).json({ categoryList });
}
if (categories) {
res.status(200).json({ categories });
}
});
};