How do I change the default browser used by Windows 11 Search?

This is the Windows Search web-result experience, not a failure of your default-browser setting. Windows 11 lets you choose defaults for apps, file types, and protocols under Settings > Apps > Default apps, so set your preferred browser there for ordinary web links. However, Windows Settings currently provides no supported option to choose another provider or redirect Start/Search results like the Fortnite result shown; that integrated result is controlled by Microsoft. Changing HTTP and HTTPS associations may not affect this specific behavior. Avoid third-party redirectors that hook Windows protocols, because updates can break them and they are outside Microsoft’s supported configuration. The reliable alternatives are to search from your preferred browser’s address bar or use Windows Search only for local apps, settings, and files. On a managed device, an administrator may be able to restrict web suggestions through policy, but that disables or limits them rather than selecting another browser.