The global Onet.xml file defines list templates for hidden lists, list
base types, a default definition configuration, and modules that apply
globally to the deployment. Each Onet.xml file in a subdirectory of
the %ProgramFiles%\Common Files\Microsoft Shared\web server
extensions\14\TEMPLATE\SiteTemplates directory can define navigational
areas, list templates, document templates, configurations, modules,
components, and server email footers that are used in the site
definition to which it corresponds.
Definition of ONET.XML
This xml file defines the element of a feature, you can have more then one element file in a feature but with different names, this file doesn't have to be named as elements but you reference it in feature.xml.
defaultdws.aspx & default.aspx
These are default pages used by web template, I couldn't find out any documentation on it.
Looking at questions you keep asking, I would recommend you to go through
Site Types: WebTemplates and Site Definitions
But in short, WebTemplates are based on site definitions, if you are creating a site definition you can create it from scratch but if you are creating a web template it has to be based on a site definition as described in this picture below,