What does ui.preferExtension actually control?

Value Behavior
true (default) If the extension is detected, use it. Otherwise fall back to MWP/QR/mobile.
false Always use MWP / QR / mobile, even if the extension is installed.

Use false only when you specifically want to force the cross-device flow (e.g., a “Use mobile wallet” button in your UI). For React Native, set this to false since there is no browser extension.