How do I create content during site installation?
There are few ways in Drupal 8 to manage site default content:


Default content module allows to export/import content via drush and store the content in the structure similar to config in Drupal core.
Umami profile in Drupal core has a custom importer which loads the content stored in CSV to be important as part of module install.
Migrate CSV plugin allows to important the content from CSV using Drupal core migration API.
Single File Components could be used to provide content with components.