The footer markup is a T4 block – a small HTML file:

html/layouts/t4/block/footer.html

in the template. Copy it to

templates/<your template>/local/html/layouts/t4/block/footer.html

and edit the copy. T4 checks local/ before the template, so your copy is used and an update can't overwrite it.

Blocks are plain HTML with module includes, so rather than typing the text into the file, add a position and keep the text in a module:

<div class="footer-legal">

  <jdoc:include type="modules" name="footer-legal" style="raw" />

</div>

Clear. Does the same local/html trick work for normal Joomla overrides too, e.g. com_content/article/default.php?