When you hover your mouse over the code he posted there will be an paper icon in it’s top right.
Click that to copy it to your clipboard, next paste it in a terminal and press enter to execute it…
On each machine you want to use different DNS’s on, you need to manually set those that were the fastest.
Because it’s a local override of the DNS setting per machine…
In my case i see this: 
> for domain in manjaro.org duckduckgo.com bbc.co.uk; do google_dns=$(dig @8.8.8.8 ${domain} | awk '/msec/{print $4}'); cloudflare_dns=$(dig @1.1.1.1 ${domain} | awk '/msec/{print $4}'); printf "${domain}\tCloudFlare DNS ${cloudflare_dns}ms\tGoogle DNS ${google_dns}ms\n"; done
manjaro.org CloudFlare DNS 63ms Google DNS 66ms
duckduckgo.com CloudFlare DNS 66ms Google DNS 39ms
bbc.co.uk CloudFlare DNS 66ms Google DNS 69ms
(In my personal setup i use my own DNS server on my own machine, so i don’t use either lol)