About Google Domains Dynamic DNS

Dynamic DNS allows you to direct your domain or a subdomain to a resource that is behind a gateway that has a dynamically assigned IP address. To use dynamic DNS with Google Domains you set up a Dynamic DNS synthetic record. This synthetic record:

Sets up an A record for your domain or subdomain that lets the Google name servers know to expect a dynamic IP.

Generates a username and password your host or server will use to communicate the new IP address to the Google name servers.

Once you set up the Dynamic DNS synthetic record, you must set up a client program on your host or server (the resource behind the gateway) or on the gateway itself that detects IP address changes and uses the generated username and password and communicate the new address to the Google name servers.
Note: Dynamic DNS only works with IPv4 addresses. IPv6 addresses are typically not dynamically assigned.

Sources from: https://support.google.com/domains/answer/6147083