once we remove the user to Confluence site since he has left company, what will happen to all the contents he has created or commented? Will these contents still be there showing his name or all the contents related to him will be removed too?
Their personal space will be put in the bin, but nothing else will be touched.
Their created and edited pages, and comments, will show as being made by an unknown user, but nothing will be deleted.
We usually recommend that you don't delete users, so their content remains attributable to them, but just remove them from the "can log in" group so that they don't count towards your user limit and can't log in to do aything.