RTK Everywhere Firmware
The SparkFun RTK EVK runs on universal firmware created by SparkFun called RTK Everywhere. You can find its source through the RTK Everywhere GitHub repo.
Make sure to check out the Quick Start Guide for the RTK EVK for a summary of how to get it connected over Bluetooth. You will also find more information about navigating through the configuration menu, updating firmware, and more in the product manual!