These three are found under ☰ → Settings → Start page & search, and are all protected against unauthorized changes — see Browser Setting Protection.

SettingWhat it controls
Set start page…What new tabs and the Home button open. Leave it blank to use the private SearXNG start page.
Set startup page…What a fresh launch opens — independent of new tabs. Blank means it follows your start page too.
Search engineWhere address-bar searches go: the local SearXNG instance (keeps searches on your machine), or an external service — DuckDuckGo, Startpage, Brave, Google, or a Custom… template.

Choosing an external search engine sends your searches to that service. SearXNG is the only option here that keeps your searches on your own machine.