I found this info interesting and useful when troubleshooting connectivity issues and just passing it along…good read 
I read this and still have no idea why or how I would want to do anything with DNS? I don’t use a VPN so maybe it doesn’t matter to me.
I follow the “if it ain’t broke, don’t fix it” rule when it comes to DNS settings. In the 30 years I’ve been using the internet I have a vague memory of fiddling with it to learn how it worked and one time I actually had to fix something.
The article allegedly is about “custom DNS” but in fact she is just pointing out that multiple configurations happen and that might be messy. In general I understand but it isn’t exactly a big problem.
It can be a pain if your trouble shooting but you should always check the local DNS setting when network troubleshooting anyway.
So let’s say I set 8 8 8.8 (Google public DNS) on my streaming device. It uses that IP as one in a list of upto three DNS servers after it checks cache etc, it never see the DNS server settings on the router.
Custom DNS (or Private DNS) is a more recent term used by phones mostly for things like DNSSEC servers (encrypted DNS), usually in the network settings but that depends on the phone and OS version and flavor.
Private DNS uses a domain name on Android streaming devices if in strict mode.
TDUK’s DNS switcher is a form of custom DNS for Android/Google TV devices which usually includes an ad blocker depending on the server you select. In turns on Private DNS and populates the target server, two different Android settings, a global setting to enable:
SNA:/ $ settings get global private_dns_mode
hostname
(can be set to “off”, “automatic”, or “hostname”)
and
SNA:/ $ settings get global private_dns_specifier
base.dns.mullvad.net
(These are my 4k Pro settings using TDUK’S DNS Switcher.)
The private_dns_mode:
off is obvious.
automatic means if a query fails it uses the default DNS network settings.
hostname is strict and it will only use the specified server.
The Android/Google TV device will use the network DNS setting to resolve the domain name to IP, and get the IP address of the DNSSEC server only. Then encryption is used for all queries.
This is usually not a problem on my device but I wouldn’t set a router up this way if it had a Private DNS option. There have been one or two times I had to turn it off on the Pro but very seldom.
I have to disable Private DNS on my phone for certain sites.
A lot of people use a VPN to hide quires from their ISPs, Private DNS does that.