Connect Linux to two networks simultaneously

I want to connect to two networks simultaneously using the Ethernet and in-built WiFi or to two WiFi networks by installing an additional WiFi adapter.

My requirement: I have two sets of devices (Group A and group B) that need to communicate with each other but not directly. There is no internet at the location. Group A devices are connected to WiFi router A and group B devices to WiFi router B (It is necessary that they should not be connected to the same router). I want the Linux device to connect to both router A and router B and act as an intermediary between the two groups. The Linux device should receive messages from Group A (through router A), process the information and send notification of the result to Group B (through router B) and vice versa.

Questions:

1. How do I setup Linux to connect to two networks simultaneously?


2. How do I identify which network a message is from and handle that message accordingly?


3. How do I specify the target network for my notification and send it to that network?