As a first step before you could dig into reverse engineer an Android application you have to fetch it from somewhere. This is not so easy due normally only Android devices download them from the Google Playstore.
Introduction
The first step in analysis of an Android application is to get the application itself from the Google Playstore. This is not so easy due the playstore only has an option to download the application directly to your device even if you are logged in with your desktop system.
Solution
Happily others had this issue too and developed a nice service which generates according a Google Playstore link a download link for the appropriate APK file (archive file of Android devices with all content like resources, binary and more).
Go first to the Google Playstore and find appropriate URL for the desired application you want to download, then go to https://apkcombo.com/de/downloader/ and fill in the previously copied URL. Now the page will generate a download link. With this link you are able to download the desired APK file from the playstore.
Next step
An APK is nothing more than an zip archive. You can easily extract all files to a dedicated folder,
