To link to external html code for use on multiple web pages (in this case the menu system at the top of the page:
Include a link to the jquery-3.5.1.min.js, followed by the jquery code to call the external html file containing the bootstrap menu code. Both of these should be in the head html section.
Begin the Body of the html with a blank div tag, with an id equal to the id called by the jquery funtion.
Now you will only have to edit the single html document (header.html) that will then update all of the pages on your website. Or you can create and then reference mutliple external files for headers, footers, leftside menus, etc.
...