As far as I know it is the default way of handling multiple DNS servers. I’d guess that at least some of the firmware running around treats them as primary/secondary, but based on my (limited) understanding at least majority of linux/bsd based software uses one or the other more or less randomly without any preference. So, it’s not always like that, but I’d say it’s less comon to treat dns entries with any kind of preference instead of picking one out randomly.
But as there’s a ton of various hardware/firmware around this of course isn’t conclusive, for your spesific case you need to dig out pretty deep to get the actual answer in your situation.
And that subset of people needs to have at least some kind of mindset to learn the viable minimum skills to even start with and a will to learn more and more and more. I’ve done various kinds of hosting as a career for couple of decades and as things change I’m fighting myself if it’s worth my time and effort to keep my home services running or should I just throw money to google/apple/microsoft/whoever to store my stuff and manage my IOT stuff and throw the hardware into recycling bin.
I have the skill set required for whatever my home network might need up to a point that I could somewhat easily host a small village from my home (money is of course a barrier after a certain point), but I find myself more and more often thinking if it’s worth the effort. My Z-wave setup needs some TLC as something isn’t playing nicely and it causes all kinds of problems with my automations, my wifi network could use a couple of sockets on the walls to work better, I should replace my NVR with something open source to include couple of more cameras around the yard and have better movement recognition and cameras should go to their own VLAN and so on.
Most of that stuff is pretty basic to set up and configure (well, that z-wave network is a bit of it’s own thing to manage) and it would actually be pretty nice to have all the things working as they should and expand on what I have to make my everyday life even more simpler than it already is. But as there’s a ton of things going on in life I just rather spend few hours gaming from my sofa than tinker with something.
That’s of course just me, if you get your reward and enjoyement on your network then good for you. Personally I think I’ll keep various things running around, but right now in this place I’m at, the self hosting, home network and automation and all that is more of a chore than a hobby. And I’m pretty sure I don’t like it.