What is the transmiting API for master in I2C for EFR32
Silicon Labs provides a simplified driver to communicate with your specific humidity and temperature sensor (Si7021).  The driver is in the board support package.
Here is the driver documentation:
https://siliconlabs.github.io/Gecko_SDK_Doc/efm32g/html/si7021_8c.html
https://siliconlabs.github.io/Gecko_SDK_Doc/efm32g/html/si7021_8h.html
Source Code:
https://siliconlabs.github.io/Gecko_SDK_Doc/efm32g/html/si7021_8c_source.html
https://siliconlabs.github.io/Gecko_SDK_Doc/efm32g/html/si7021_8h_source.html
The source code appears pretty straight forward.