How CNAME records work
A CNAME record is stored in your domain’s DNS settings as a pair of values. One value identifies the alias you’re creating the record for, which is typically a subdomain like www or mail. The other value identifies the domain the alias should point to.
With Google, a CNAME record can direct the web address www.example.com to a web site built in Google Sites for the domain example.com. Another CNAME record can direct mail.example.com to the Gmail sign-in page for example.com. And CNAME records can be created for the domain’s other services, as well.
In this way, CNAME records make it easy to run multiple services from one IP address. Each CNAME record associates a service with a domain name, not a physical IP address. The physical IP address is instead identified by your domain’s A record. If your IP address changes, you only have to change the A record, not each CNAME record.
Like all DNS records, CNAME records are stored by your domain host and therefore must be changed with the host, not in your Google Admin console.