Applying a common footer across all pages

There are dozens of ways you can achieve this, with or without server-side scripting:

Below is an eaxmple without server-side scripting, just using a javascript file.

First you would need to set up a container on each page where you want the navigation to appear, along with a link to a javascript file, in this case 'navigation.js': You have to initially insert some kind of code on each page if you use a server include script like php, so its no extra steps.

<nav>

<ul class="mainNav">

<script src="navigation.js"></script>

</ul>

</nav>

Then in the navigation.js file you can store the 'linkUrl' and 'linkText' in an object array then loop through it using the forEach function:

const mainNav = document.querySelector('.mainNav');
const navigation = [
{linkUrl: 'link_1.html', linkText: 'Link 1'},
{linkUrl: 'link_2.html', linkText: 'Link 2'},
{linkUrl: 'link_3.html', linkText: 'Link 3'},
{linkUrl: 'link_4.html', linkText: 'Link 4'},
{linkUrl: 'link_5.html', linkText: 'Link 5'},
{linkUrl: 'link_6.html', linkText: 'Link 6'},
{linkUrl: 'link_7.html', linkText: 'Link 7'},
];
navigation.forEach(function(nav) {
mainNav.innerHTML += `
<li><a href="${nav.linkUrl}">${nav.linkText}</a></li>
`;
});

I prefer to use a php include because php is what I use by default but  javascript is an option if you dont have a server-side set-up, along with the other solutions so far provided.

As Jon and Nancy tell you, file inclusion is the most flexible and common way to use and share page fragments when creating a website.
Jon points to Dreamweaver's Templates, which will certainly ask you to start over, to support all your pages on this technique, but you can also use Library elements, without starting from scratch.
the difference between library items and file inclusion comes from the difference in technology used.
The inclusion is based on the server side, the library elements the code of the pages.

so, any change will require to place online only the file included on the server....... whereas if you change a library element you will then have to upload all the pages that contain it again

https://helpx.adobe.com/dreamweaver/using/library-items.html

I highly recommend using server-side includes because they are powerful, efficient and easy to update.  Open your include file, change it, save it, upload it to server.  You're done!  And includes will work with any editor.  

In contrast to includes, DW's proprietary Templates and Library Items only work in DW and only on the local site files.  This means you have to re-publish your entire site every time you update those elements.  That might be OK for small sites but not ideal for large ones.

I use PHP includes for everything because that's the server technology my hosting plan supports. 

https://alt-web.blogspot.com/2015/07/server-side-includes-with-php.html

The easiest ways would be with .PHP or .SHTML includes, especially if the site is larger than about 50 pages or so. That way, you only write one "include file" with just the html of the repeated region you're replacing, tack a tiny include script into each page where you want it to appear, and the server replaces the script code with the include file code. It's super easy with both .PHP and .SHTML (no php installation required).

If you can't use those technologies, due to server limitations, DW has a handy tool called .DWT Templates that would do what you want, but would require basically starting the site over...

https://helpx.adobe.com/dreamweaver/using/dreamweaver-templates.html