Web Development

I commented on another comment of yours too, but I think the easiest ways to do this would be with web components or with PHP includes

If you want to dive deeper, you can learn a javascript framework like Astro or Svelte which have more of a learning curve but are better at organizing larger or more powerful websites/web apps

This is normally done on the server, by whatever tool is building the final html pages. If it's just a static website (doesn't take user input, glorified brochure), then one might use a static site generator (eg Jeckyll), each of which have their own mechanism for sharing common snippets. If building a more dynamic website with a database backend, then one would be using other tool (eg. Ruby on Rails), which would also each have their own mechanisms for sharing common snippets.

I'm still new to this, so I might be getting things backwards, but I think I've recently done what you're looking for.

I'm making a web app for a small music festival, and was duplicating the page links on every page, and having to update them all every time something changed. I found a technique online to put the links into a header.js file, then just call the script in every new page. I did the same with the footer, and created a template for a blank page with the html that couldn't go into the header.

I'm not near my computer to get the site with the instructions, but my Github is here if it helps. Just bear in mind that I'm new, so might not have done it in the best way

https://github.com/Tippon/Cwmfest-test