The menu items (custom buttons) are linked in the object link panel using click action > Scroll to element. When clicked, the viewport will scroll down to the targeted element.

A tutorial on how to create custom buttons can be found here

In the object position panel, you can set an element or group alignment to Sticky and choose to which viewport-border.

Sticky does not position on the page borders but to the viewport borders. A sticky element always stays inside the viewport while scrolling and does not respond to the safe zone.

In this example the position of the horizontal menu-bar is fixed to the top of the viewport regardless of how far the user scrolls down.