It's called a secondary DNS server. Like, literally the reason it exists. I guess it's still on the line towards knowing what TF you're doing.
Every DHCP server offers at least 2 dns server options.
Care to elaborate? Last time I tried to set the secondary DNS as backup while keeping the pihole filtrering there was no real way to do it without having two piholes. Even the pihole developers said as much
Right, we are on the same page but from experience, setting DNS 1 to the pihole and DNS 2 to for example 8.8.8.8 in your DHCP router will make the pihole useless. There are dozens of similar threads across different forums saying the same thing but if you know how to do it please let me know... Example: https://www.reddit.com/r/pihole/comments/864oli/secondary_dns_setting/
Well, after working as some form of systems engineer for the last 17 years, including quite a few where some of my primary responsibilities were looking for DNS servers, this is literally the first time I have come across this.
Also not quite sure what they're doing because my Debian, 2x Windows, 3x Android and occasionally Apple clients never bypass my primary DNS setting. Neither do the server farms I run at work. So who knows.