UART vs Transmit() and uPY

Hardware is Xbee 3 802.15.4 module (and latest FW). From the perspective of data flow direction being data coming in on UART and then RF transmit, is there actually 2 different paths to get data into the module to be RF’d out? Meaning one of the ways is as the module is designed to do, whatever comes into the module via the UART pins is then RF’d out. The other path is uPy code flashed into the module (ATPS = 1 so main.py starts on power-up) and within that uPY code the payload in transmit(dest,payload) function is then RF’d out. My testing says all this is true. If that is the case, what or how really does the uPY transmit() function do this? Does it internally in the module put the payload onto the UART so in reality there is only 1 path ie UART IN then data RF’d or is there something else in the module that is there specifically for the transmit() function? Thanks

The primary UART is for sending data in and out of the radio. Micro Python sends data directly to the RF function of the radio via the ()Transmit function and the Digi firmware.