There is no ‘default sidebar area’ in WordPress. It’s entirely up to themes to register whatever widget areas they need. So you will need to use register_sidebar() to create the areas for widgets to be added (‘sidebars’). Then in your theme you need to use dynamic_sidebar() to output those widgets.
The get_sidebar() function is the equivalent of get_header() and get_footer(), but for sidebar.php. As described in the documentation:
Includes the sidebar template for a theme or if a name is specified then a specialised sidebar will be included.
For the parameter, if the file is called “sidebar-special.php” then specify “special”.
If you do not have a sidebar.php file then get_sidebar() will output a default set of widgets. This is purely for backwards compatibility purposes, as it used to be how sidebars worked. This behaviour is now deprecated and you should properly define your sidebars in your theme.
Please note that this is all described in great detail in the documentation. If you are developing a theme there is no excuse for not reading the docs.