You can use a CName to map your desired sub domain to your Netlify site as per their documentation:
Mapping an existing sub-domain
The important thing is that both ends (Google Domains and Netlify) need to know about the new domain.
In summary you need to configure your external DNS provider to point your domain to Netlify.:
InOn your Netlify control panel, select the Custom domains"Custom domains" panel, select Check"Check DNS configurationconfiguration" next to the custom domain to open customized details about the DNS records you need to configure. The instructions will vary depending on the type of domain or subdomain.
To point a subdomain such as blog.petsofnetlify.com to your site on Netlify, you must first create a CNAME record with your DNS provider.
For example, if your site’s domain is blog.petsofnetlify.com and your Netlify subdomain is brave-curie-671954.netlify.app:
- Find your DNS provider’s DNS record settings for your apex domain,
petsofnetlify.com. - Add a CNAME record with your subdomain,
blog, as the host. - Point the record to your Netlify subdomain,
brave-curie-671954.netlify.app. - Save your settings. It may take a full day for the settings to propagate across the global Domain Name System.
Once you've completed those steps with Google Domains, you then need to complete the steps in the Netlify control panel to assign the custom domain to your site:
Select "Add custom domain" at the bottom of the "Custom domains" panel, and enter your domain name. Select "Verify".
Pressing verify will check that the DNS settings you've entered in Google Domains are correctly configured and pointing at the Netlify site - if they are, then your new domain should be configured and serving pages as expected.