>> "1. What is the user of client.wyc to include in the application "
Are you asking what's the *use* of client.wyc? It includes information about your app. We have FAQ articles that cover this stuff: https://wyday.com/wybuild/help/faq.php#in_client_wyc
>> "2. I am currently using DropBox for my customers or partners to download the files. Can i use the same as well ?"
Yes, see: https://wyday.com/wybuild/help/walkthrough.php
You can use any site that's http, https, ftp, or file:///
>> "3. Why we need to upload wyserver.wys to FTP ?. What is the use of this ?"
It includes information about your updates. It's part of your updates. So, upload it to your servers. See: https://wyday.com/wybuild/help/walkthrough.php