Brave does not replace (meaning, display in-situ) third-party ads. Those are blocked, as you stated, for ethical and security reasons. Brave offers an opt-in feature of the browser called 'Brave Rewards' and 'Brave Ads'. This feature enables users to opt-in to ad-notifications (displayed as native prompts on the device; not shown on the pages you visit). Ads are displayed on every 4th new tab page for participating users (again, no ads on publisher properties). The user consents to these, receives 70% of the revenue for their attention, and sets threshold limitations for how many ads can be displayed in a given period. Brave has never replaced ads on pages; that would be highly unethical.