How to get Media by REST Queries?

[NammDev]

I am struggling with how to retrieve media values through REST queries, as the previewImage field is related to the media collection.

When I access the data through the API endpoint, it displays the previewImage field as an object containing all the fields of media collection:

  "id": "647dc6314fd949fdddbe9fbd",
  "title": "This is a title",
  "previewImage": {
    "id": "647dc5f14fd949fdddbe9f87",
    "alt": "preview-img",
    ....
  }
  .....

However, when I use a function to retrieve the same data, it only shows the id property as a string.

id: '647dc6314fd949fdddbe9fbd',
title: 'This is a title',
previewImage: '647dc5f14fd949fdddbe9f87',
...

I would like to know why this is happening and how I can properly retrieve the previewImage data as an object with all properties of media collection.
As a new programmer, I am not yet familiar with Graphql & Apollo Client like in the demo repo. Do I need to learn about Graphql & Apollo Client in order to retrieve Media using REST Queries?

Here is my function

export const fetchArticle = async (slug: string): Promise<Article> => {
  const query = {
    slug: {
      equals: slug,
    },
  }
  const stringifiedQuery = qs.stringify(
    {
      where: query,
    },
    { addQueryPrefix: true }
  )
  const res = await fetch(`${process.env.NEXT_PUBLIC_CMS_URL}/api/articles${stringifiedQuery}`, {
    cache: 'no-store',
  })
  const data = await res.json()
  return data?.docs[0]
}

[NammDev]

Please some one can help me?