There are two UARTs on the 20737X devices (SIP module and SoC).
One of these UARTs is dedicated to programming the device and is called the HCI UART; this UART is not available to the application.
The HCI UART is on physical pins 18 and 19 per CYW20737S Bluetooth Low Energy SiP Module Technical Reference
The other UART is referred to as the peripherhal UART (PUART) and can be configured and used with an external sensor/MCU based on the GPIO limitations outlined in the WICED Smart Hardware Interfaces
The logical (firmware based) pins used by this port on the TAG3 evaluation board are P32/P33 as noted in /WICED-Smart-SDK/include/Platforms/BCM920737TAG_Q32/platform.h: