Ecosia is a search engine that aggregates search results from multiple other search engines. The ad revenue from our searches funds the planting of trees worldwide. With over 200 million trees planted so far, Ecosia have learned to be fully transparent about their projects, and financials which are available right on that website. Set it as your default search engine, and start planting trees. They also recently released a Chromium based browser if that’s your thing - TechCrunch article for reference.
No such assumption is made. Ecosia is clear when an entire project fails, and takes into account tree survival rates when they claim to have planted over 200 million trees. It’s all clarified on their website. Regardless of all of that, they take all ad-revenue to reforest and I’d say that’s a better deal than Google or MS.