So, to sum up . . .
1. Paste this into your site’s Footer Code Injection:
<script>
document.querySelectorAll('[data-ad-placeholder="ads-1"]').forEach((elem) => {
elem.innerHTML = "... your ad html ...";
});
</script>
(making sure any classes used within the "... your ad html ..."
portion use single quotes, not double quotes.)
and then . . .
2. Paste this into an HTML card on a page:
<div data-ad-placeholder="ads-1"></div>
That way any time you want to update your content for that snippet (to apply to every instance all across your site) you can edit that snippet once in the Site Footer Code Injection from the admin area.
Thank you, @Kevin and @Stromfeldt !!