they use subdomains of 1e100.net as names in their network
yebyen:~$ host google.com
google.com has address 172.217.10.46
google.com has IPv6 address 2607:f8b0:4006:803::200e
...
yebyen:~$ host 172.217.10.46
46.10.217.172.in-addr.arpa domain name pointer lga34s13-in-f14.1e100.net.This allows flexibility in infrastructure — you can swap machines in and out (e.g. by updating public DNS records, updating the machines’ IP addresses, or adding them to (or removing them from) a load balanced pool behind a reverse proxy). But you still need a way to reference individual machines regardless of whether they’re serving or not.
That’s where domains like 1e100.net come in — a system of concrete (non-abstract) references to specific machines in your infrastructure.
$ ping google.com
PING google.com (172.217.7.206) 56(84) bytes of data.
64 bytes from iad30s10-in-f14.1e100.net (172.217.7.206): icmp_seq=1 ttl=48 time=0.676 ms