Access the customizer
Most of the controls for configuring the header of your website can be found within the WordPress customizer. You can access that control panel by either clicking the link within the admin toolbar (when logged into the site) or from the admin sidebar at Appearance -> Customizer


More from wordpress.org: https://wordpress.org/support/article/appearance-customize-screen/
Site title and parent unit
The Site Information panel within the customizer provides the necessary tools to display the site title within the header. Within this panel there are also options for adding a parent unit and linking that parent unit name to a separate website.
Be sure to press the “publish” button to make the changes permanent after you are done with your edits.

Adding a navigation bar
To display the navigation bar within the header, you need to assign a menu to the menu location labeled main menu. That menu will also need to contain at least one link in order for the menu bar to be displayed.

Controls for building a menu and assigning menus to a menu location are also available in a different part of the WordPress admin at /wp-admin/nav-menu.php
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There are advantages and disadvantages for using the customizer (as pictured above).
- For making quick edits to an existing menu, or for the ability to see the results of your edits in real-time, the customizer is the best tool to use.
- A visit to the customizer is also usually needed to configure other aspects of the site as well. (One less click!)
- However, if your plan for the navigation of the site includes the need for a call-to-action button, you MUST use the interface at
/wp-admin/nav-menu.php
Additional things to note:
- If the website was to be used as a landing page using the “no navigation” state of the header, you can leave the main menu location unassigned as pictured below. The site will still function normally in all other regards.
- The “no navigation with buttons” header state can be achieved by adding only CTA buttons to a menu that is assigned to the main menu location.