How do i get api for MI Band 6 watch faces?

Question

I want to create an application that will display collection of all mi band 6 watch faces available to download.I saw some of the application on google playstore that already had these feature .I googled on how they are doing it but found nothing relevant to it.I am sure they are accessing the band watches from some website. Do anyone have any idea how to achieve this task? Attaching a link to one of such app from playstore for better clarification. https://play.google.com/store/apps/details?id=com.bandfaces.miband6