1: There is no one-to-one correspondence between the communication interface and the communication protocol; for example, the RS485 interface can use Modbus, Profilbus, TCP/IP and other protocols.
2: Different protocols are not compatible, Modbus, to connect to the Profilbus bus, you need a switching device. Search specifically for Modbus to Profilbus.
3: The communication protocol can be defined by yourself, the rules are set, and the connected devices can communicate.
4: It’s the same as question 3. Make an agreement and everyone can communicate. For example: you can use the query method, the PC sends an application to the device, and the device receives the application and sends the data to the PC.