To install apps from an unknown source please follow below the path:
Setting method:
realme UI 3.0 & above versions: Settings > Search "Installation sources" in the search bar > turn on the corresponding application switch to install the app.
realme UI 2.0 & 1.0: Settings > Security > Unknown source installations > turn on the corresponding application switch to install the app
realme UI R & Go & T & S & U Edition: Settings > Search "Install unknown apps" in the search bar > Install unknown apps > turn on the corresponding application switch to install the app.