It's based of the first part of the URL that will be concat with a final one of a specified lenght in order to find a patter in the resulted page.
For example we can scan instagram's post with the base url 'instagram.com/p/' and an alphanumeric string and then find the direct url of the photo to download the photo.
It's possible to specify the string alphabeth and the initial string. It's possible to specify a delay between the requests to avoid ban.