Could someone explain the RS485 section of the EVALSTPM3X-3PH board?
STM8 RX pin can receive data either from USB via Q1 or RS485 via Q4, will both affect the STM8 RX pin  via the isolator.

STM8 TX pin can transmit data via the isolator, it goes directly to USB RX pin and via Q2 to RS485 transmit enable pin.