Regarding Billing Account

I have a question about linking a billing account. If I link a billing account to my project, will I automatically be moved to a paid (Tier 1) plan and start incurring charges for generating responses? Or, if I use the Gemini 2.0 Flash model within the free tier limits (e.g., 15 requests per minute), will it remain free, and I’ll only be charged if I exceed those limits?