I’m certainly no expert in this area, but I believe this is configured in the browser itself? https://support.mozilla.org/en-US/kb/firefox-dns-over-https

Perhaps there’s another way to do it system-wide, but I wouldn’t know it.

Yes, that pop os. As luck would have it, Linus installed it during a very brief period where the steam package in their repo was broken. This is not a common occurrence, and I have never heard of it happening before or since.