PO9O is a start page, not a search engine. You type a query, pick an engine, and press Enter. The query goes straight to that engine with its AI layer switched off. Queries are never kept and there is no account. Your theme, engine and new-tab choices are remembered in this browser and written into the page address, so a bookmark carries them to another machine.
Each engine needs a different trick, because none of them agree on how to turn the generated answers off:
udm=14. Classic links, no AI Overview. Undocumented, so it could change.noai subdomain, which drops Assist and AI-generated images.summary=0 suppresses Answer with AI.Engines with no dependable off switch are left out on purpose: Bing, since it dropped its Copilot toggle, and Ecosia, whose control depends on your region.