A web search box located in the middle of the Windows 11 desktop might not be a highly requested feature from users, but Microsoft is trying it out anyway with Insiders in the Windows 11 Insider Preview Build 25120, just released to the Dev Channel.
It's only a test for now and Microsoft is using it to gauge feedback from testers on the Dev channel. New features like this aren't guaranteed to turn up in a build until they reach the Beta channel. (Microsoft explained how it tests new Windows 11 features with Insiders in a February blogpost.)
The search box is by default powered by Bing, Microsoft's search engine. However, early tests by Windows watchers indicate that using the Windows 11 desktop search box automatically opens results in Microsoft Edge, ignoring the default browser users have configured for Windows 11. Again, it's just a test feature, so ignoring user configurations might not be a big deal unless this behavior carries over to beta and later.
Other Windows observers wondered how difficult it will be for users to switch the search engine used by the search box. It appears users can't change the default search engine.
The search box being tested on the Windows 11 desktop is basically the search widget from the Windows 11 widgets panel. Windows 11 introduced a new translucent widgets slide-out panel to display weather and various content, including a search box. The panel can be opened from a widget button -- a blue square with a white cross -- in the taskbar.
Microsoft doesn't offer answers to these questions in its blogpost, but does explain that in preview builds going forward it is exposing "lightweight interactive content on the Windows desktop" in the Windows 11 Widgets board.