Explain in detail about the google forms scan.

The spammer in question is abusing a Google Forms feature called Response Receipts. If you create a Google form and turn on response receipts, then whenever anybody fills in and submits the form, a copy of the filled form is sent to the email address they provide in the form. This response receipt is sent from the Google service account forms-receipts-noreply@google.com and since it is sent by Google itself, it is also signed by google.com.

So all the spammer has to do is create a Google form spam with the body of the form containing the content of their spam email. Then they simply submit the form with the victims' email addresses and sit back as Google delivers their spam for them. Note, however, that when I tried replicating this, the response receipt email landed in my spam folder, which means Google does attempt to detect such spam emails. The spammers must have figured out a way to bypass gmail's spam filters for the email to land in your main inbox.