Ah, ok, then snippets won’t do what you want there. Ghost doesn’t have any functionality for doing things like that server-side because content is stored and rendered as static html.
It sounds like you’d be better off adding a placeholder to your content, eg a html block containing <div data-ad-placeholder="ads-1"></div>
or similar and then in your code injection footer having something like this:
<script>
document.querySelectorAll('[data-ad-placeholder="ads-1"]').forEach((elem) => {
elem.innerHtml = "... your ad html ...";
});
</script>
That way any time you wanted to update what your ad content is you can edit the code injection from the admin area.