1.) standard POP method . . . POP Mail download
Roundcube doesn't use the POP protocol to retrieve email.
2.) If I view a mail in RoundCube, will the same email be available to download
Yes, if you access with roundcube, the message will stay on the server.
3.) Or Does Roundcube keep it?
Roundcube, with caching enabled, keeps a copy of the message in a database.
4.) what happens if i add the mail to a folder in RoundCube, does RoundCube keep a copy of it and allow POP Mail download still ?
Yes, because as stated before, RC doesn't use POP so everything is done server-side. So changes you make will be available outside of the RC interface in other programs.
Hope that answers any questions.