When building custom mobile or web applications that aim to replicate the look and feel of Salesforce Lightning Experience, the User Interface (UI) API is the architecturally recommended choice.

The UI API is specifically designed to provide the metadata and data needed to build high-fidelity user interfaces. Unlike the standard REST API (Option B), which returns raw record data, the UI API returns both data and metadata in a single response. This includes information about page layouts, field-level security, picklist values, and localized labels. By using the UI API, the mobile app can dynamically render fields according to the user's permissions and the organization's layout configurations, ensuring that the custom app stays in sync with changes made in Salesforce Setup without requiring code updates in the mobile app.

Connect REST API (Option A) is primarily used for Chatter, Communities (Experience Cloud), and CMS content, and while it is useful for those specific social features, it does not provide the layout and record-level metadata required for a full CRM interface. The UI API is the same underlying technology that powers the Salesforce mobile app and Lightning Experience itself. Therefore, utilizing this API allows NTO's developers to build a native app that perfectly mimics the Lightning Experience while reducing the amount of custom logic needed to handle complex Salesforce UI requirements.