If you have a long menu and want to keep all the menu items in a single line, then you can do the following:

From your WordPress Dashboard, go to the Appearance > Customize > Header > Menu > Primary Menu section.
Switch to the Keep Menu Items on One Line option. The part of the menu that didn't fit in the line can be accessed by clicking on the more options icon ( i.e., three vertical dots).
Click on Publish.

It is shown in the image below:

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