This question has been asked before, including this one not that long ago: Is asking about search engines ( startpage.com + duckduckgo.com) On Topic on superuser?. Search engines are generally web apps, and have been off-topic on that basis.
Starting fairly recently, many of the major browsers require "installing" search engines that will work from the search window. You can still use search engines like a web app (go to its URL), but the ones installed in the browser's search engine list and selectable from the search box pull-down are more like add-ons in that they function like an integrated feature of the browser.
Knowledgeable users may understand that behind the scenes, the search engine still operates like a web app. But to the people asking questions, it appears to be an integrated feature of the browser. We can just have a rule that search engines are off-topic, so it's clear for reviewers. But we may end up having to explain how browsers work to a lot of questioners so they understand why their question is off-topic, but a seemingly similar one about an add-on isn't.
Should we reconsider treating search engines like add-ons, installed add-ons being on topic?
https://www.google.com/search?q=<searchstring>for google or
https://duckduckgo.com/?q=<searchstring>&ia=webfor duckduckgo and passing those URLs on to the search engine?