Downloading from external sources

Before downloading books from an external source (e.g. Amazon), I'd like to understand the process.  I know that the Kindle app no longer downloads, so I will need to log on to the regular Amazon site.  Since the Nova 3 Color I have is Android-based, I assume that the process will be the same as in any other Android device.  Assuming that that is correct, which directory should books be put into?  If that's not correct, what is the actual process?  I assume that whatever the process is it will be identical for all sources, not just Amazon.  Correct?