A good question! You can apply this to anything you want across a site not just footers
Although, on the point of copyrights, I'm of the opinion that those are a feature of corporate websites and are not needed on personal sites, each to their own though!
To answer your question, if you use a static site generator you can do this pretty easily - Although I cant help much with that.
If your not using a site generator, include a script at the end of each page that updates the footer, then you just have to update the script to update every page. Its very fast to load, the downside is it does not update if people have scripts turned off.
HTML
<footer id="foot">:grin:efault footer text for people without scripts</footer>
<script src="footUpdate.js?v=0"></script>
Script
var footText = "© 2022 - My Rad Site";
var foot = document.getElementById( 'foot' );
window.onload = function() {
foot.innerHTML = footText;
}
OR You could also put an iframe in your footer to a default footer page. This is a bit slower as it needs to loads multiple pages (your main page and the footer page) but it still works if people have scripts turned off.