The default front end theme in Drupal core since version 9.4 is Olivero. It is a clean, accessible, modern theme designed to showcase what out of the box Drupal can look like.
Before Olivero, the default was Bartik, which had served as the face of Drupal since version 7. Olivero replaced it with better accessibility, improved responsive behaviour, and a more contemporary visual style.
On the admin side, the default theme is Claro, which replaced the older Seven theme. Both Olivero and Claro are actively maintained and suitable for real projects, though most production sites build a custom theme tailored to their brand. Olivero also works well as a base theme or reference when you are learning how modern Drupal theming is structured.