You can use CSS to make every piece of your page Sticky. A menu bar then remains at the top when scrolling. You give the section a class or ID name and you add position: sticky. A CSS instruction that is fairly recent. Older browsers can not do this.
The Sticky module does the same with more options.
Look at the menu.
It is a third party module.
Look for the ID or class
Your part is being wrapped into a new div.