so I think Modbus is a protocol and RS485 is the interface. What does the interface mean?On Industrial level there are typically two types of RS485 protocol. "Profibus" and "Modbus". Siemens, Rocwell, ABB, and many other PLC/DCS related companies use these as their network communication between CPU(s) and I/O(s). These protocols can be over medium such as 2-wire copper, fibre optic or CAT5 or CAT6 for Ethernet.
If you want to learn RS485 (with Modbus) itself bare bones format, I would suggest using Arduino and MAX485 chips. There are many Libraries available. Search for SimpleModbusSlave and SimpleModbusMaster.