A slave DNS server refers to an alternate source to obtain URL and IP address combinations. This is useful when there is a master DNS server where the entry list is maintained. In version 6.2 and later, FortiGate as a DNS server also supports TLS connections to a DNS client. See DNS over TLS for details.

How to replicate dns record to a second dns server Jun 18, 2013 [SOLVED] Any reason to have multiple DNS servers? - Spiceworks Dec 27, 2014 add two different server for dns and domain controller

Best free DNS servers of 2020 OpenDNS. 208.67.222.222. Owned by Cisco, OpenDNS has two free options: Family Shield and Home. Family Shield is good for parents who want to make sure their kids can't access inappropriate content.

So, with 2 DNS servers on the domain, clients will use the first one listed[as you title primary]. If that server is unavailable, the client will look at the second one listed[or as titled Get Started | Public DNS | Google Developers Jun 25, 2020

I would suggest having 2 physical DNS servers/DC's and then have a third that is virtual ideally with failover. If you have a critical event occur within your server room or other environmental disaster you have a functional DC.

DNS over TLS vs. DNS over HTTPS | Secure DNS | Cloudflare