What are the default SXA modules available for CMS user to build website?
When creating a new site within your tenant, you can (de)select the modules you wish to add to your site. These are the default modules available (SXA 1.9) when adding a new site:
After your site was created you can always add modules which were not selected when the site was created.
Add modules to tenants and sites using context menu script (Add Tenant/Site Module) or bulk module installer. When you create a new SXA tenant or site, the wizard lets you select the modules that you want to include. When you have an existing tenant or site and want to add modules later, you can add modules to a specific tenant or site or use the SXA bulk module installer to install modules for multiple tenants/sites.
Please look at the documenation: https://doc.sitecore.com/developers/sxa/19/sitecore-experience-accelerator/en/add-a-module-to-an-existing-tenant-or-site.html
In addition, you will not get all the modules in the module tab. Default SXA modules include the not only rendering sections, but JSON site setup, and grid systems as well. SXA modules are stored in the Feature or Foundation folder under the system node -
/sitecore/System/Settings/Foundation/Experience Accelerator/ /sitecore/System/Settings/Feature/Experience Accelerator/