How to update one collection when changes occur in another?

Question

MongoDB

I have 2 related collections. When filling in the data in the form, I select the list loaded in advance, where I select the necessary IDs, when filling out the table, I send the data to the server and process it like this:

const Category = require('../models/category.model')
const Tag = require('../models/tag.model')
const slugify = require('slugify')
module.exports.createTag = async (req, res) => {
  try {
    const tag = new Tag({
      premium: req.body.premium,
      categories: req.body.categories,
      title: req.body.title,
      tag: slugify(req.body.title),
      description: req.body.description,
      name: req.body.name
    })

    await tag.save()

    req.body.categories.forEach(async item => {
      const category = await Category.findById(item._id)
      category.tags.push(tag._id)
      await category.save()
    })

    res.status(201).json({message: 'Тег успешно создан'})
  } catch (e) {
    res.status(500).json({message: 'Ошибочка'})
  }
}

In the model they are connected like this:

tags: [
    {
      type: Schema.Types.ObjectId,
      ref: 'tags'
    }
  ]
  categories: [
    {
      ref: 'categories',
      type: Schema.Types.ObjectId
    }
  ]

How can I change another collection when updating data? Here I have 2 options selected from the list, but how can I do this to overwrite it? If I make one selection from the loaded list, then everything will be saved as it should, but the previously selected item will also be present in the other collection