How secure is the data transmission between nodes? is safer to use in industrial applications?
I need to replace the RS485/modbus communication in an automobile industry
Rather than trying to explain the nuances of ROS 2 security, I think its better to mention that ROS 2 uses the DDS standard for its security paradigm. If you're working on something important like automotive, you should probably look into the actual specification itself, DDS Security (https://www.omg.org/spec/DDS-SECURITY/1.1/About-DDS-SECURITY/).
It includes the big things you think of like encryption, authentication, access control, and logging. Whether the provided DDS security is right for your application is largely up to your specific requirements and industry norms, but DDS does provide good security for what its meant for.