How does DNS domain verification work?

Im reading this google doc on how to hook up app engine to a custom domain (a domain I purchased through a different registrar)

I get to a point where I have to "Supply the domain name ("example.com") and click Verify. This opens up a new tab titled Webmaster Central." and walk through some prompts to prove I own the domain.

After doing this "domain verification is automatically re-confirmed about every 30 days".

What exactly is "domain verification"? Is there like a domain verification protocol that all DNS registrars must support? What communication is happening between google and say godady or AWS (route53)? Is there a special type of DNS record specifically for verification?

I don't understand whats actually happening to prove I own the domain and if this process is standardized or each DNS provider has their own solution/quirks for doing this.