On which Stack Exchange site can I ask questions about search engines like Ecosia?
If you can phrase it as a question that can be given an objective answer within 1-5 paragraphs, then Sustainability.SE is the network site for you.
If the question was "is ecosia.org trustable?", that's likely to get put on hold until it's improved.
If, however, the question was: "are there any independent verifications that ecosia.org is using (almost) all its ad revenues for tree planting", then that could be the start of a good question, if you then were to add a few sentences about what ecosia is, and what it claims to do. Remember, we want questions to be self-contained, so people don't have to go read another webpage elsewhere, to understand the question you're asking.
Another viable question might be something like: "is the tree-planting done by ecosia actually contributing to sustainability, by some objective measure?"
And again, a handful of sentences about them and what they are doing, and where & how they are doing it, will help people understand the context, without having to go elsewhere to find enough context to be able to answer the question.
The idea is that only one person should have to read and summarise all the context that allows the question to make sense: and that one person is the question-asker. That's much more efficient than asking a dozen (or more) other people to each do all that reading and summarising for themselves, before they try to answer it.