How do you fetch instagram?

I see some websites that can fetch an instagram profile and get all their posts, reels and videos by simply typing the username and I don't understand how they do it because it's so fast.

I've tried scraping instagram's profile by doing this:
const fs = require("fs")
const url = "https://instagram.com/instagram"

async function Scrape() {

    const headers = new Headers()

    const res = await fetch(url)
    const data = await res.text()

    fs.writeFile("result.html", data, err => {
        if(err) {
            console.log(err)
        } else {
            console.log("file written successfully")
        }
    })

}

Scrape()

But in result.html I only get a sorry there's nothing here page. How do they do it?

For example go to https://igdownloader.app/ and type https://instagram.com/instagram
IGDownloader.app
IGDownloader is an Instagram downloader that helps you to download videos, photos, stories, Reels and IGTV from Instagram. Support iPhone, Android.
Instagram Downloader: Videos, Photos, Story, Reels, IGTV - IGDownlo...
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