It's both. Free tier users get to use what's left over from Google's capacity. They pay with their data, Google uses their inputs for training. Paid tier user

It's both. Free tier users get to use what's left over from Google's capacity. They pay with their data, Google uses their inputs for training. Paid tier users pay with money, Google doesn't use their inputs. They get priority when capacity is running out (like right after a launch as happened here).