Confused about Google’s Cloud Platform costs

I’m having to migrate to Google’s "Web Risk" Lookup API after the terms of use were changed for their "Safe Browsing" product. Clicking the "Get started for free" button on the Web Risk overview page takes me to a Google Cloud Platform sign up page.

Reading the FAQ document related to this page is the source of my confusion about costs.

It says that new sign-ups get a 90 free trial, and $ 300 dollars of credit at the top of the document.

But the End of the Free Trial section says that once the 90 days are up, or the $ 300 credit has been spent:

To continue using Google Cloud, you must upgrade to a paid Cloud Billing account

To my mind this sounds like there’s a recurring subscription fee for Google Cloud regardless of whether you use any of the services therein. Is that an accurate interpretation? In other words, do you have to pay some sort of (monthly / yearly) subscription simply for having a "paid Cloud Billing account"?

Or is this description merely a way of indicating that use of some Cloud services may incur a fee once the value of the $ 300 dollar credit / 90 period has been used up?

I’ve been trying to find a clarifying statement about this, but haven’t been able to track anything down.

Thank you.