What is DDNS username and password for?

How do I set up DDNS with Google Domains?

I would like to set up DDNS on my VPS with Google Domains. I've set it up and set up an A record but not able to see any changes in Namecheap DNS.

Google Domains isn't a DNS service, it is a DNS service provider. You need to make sure you have the correct type of DNS records (A records for Hosts and AAAA records for IPv6 Hosts) and set them up in the correct way for your own domain.

The instructions provided by Google Domains are detailed on their DNS support page.

Does Google Domains have DDNS?

When I create a Google Custom Domain, can I then configure DNS? I thought the domain is created by Google and you can manage that on Google Cloud DNS. Then I want to use some 3rd party DNS (say Route 53), so if I am using this, can I point my custom domain to Google Domains and how? There is no support for using Google's DNS servers for anything outside of resolving www.google.com in your own DNS records. In fact, their support page says it explicitly:

You cannot create records in our DNS for any other domains, as the. data you enter will not be available for anything other than Google. products and services.