Free plan, no card: see everything, including what Optimize would change. Add a card when you want Optimize to make the changes.
Nothing is sent to Google until you add a card. Adding one starts your plan, and the 5% fee applies to spend from that day. If you never add a card, nothing is charged.
The full detail: Pricing.
See it on your own accounts
Free plan, no card: see everything Optimize would change before you pay.