Wireless communication shields, how do they work?

I would like to better understand how wireless shields are working.
Could someone suggest a documentation.

In particular, I want to understand:
how shield understands that message is received?
what if there are two transceivers? How does shield know that there are two messages in the air?

This communications 'shield' (aka communications peripheral board) implements hardware and software to transmit and receive data. The techniques involve multiplexing and modulation. For cases where there are multiple transceivers, there are communications techniques that allow two transmitters to send their own independent messages without interfering with each other. Look up words like 'multiplexing', and acronyms like 'DSSS' and 'OFDM'.

What is the context for "Look up words like 'multiplexing', and acronyms like 'DSSS' and 'OFDM'."?

Should I search for hardware or software solutions?

afedorov:
What is the context for "Look up words like 'multiplexing', and acronyms like 'DSSS' and 'OFDM'."?

Should I search for hardware or software solutions?

The context is ...... you must learn electronic communications techniques in order to understand how messages from 1 or more transmitters can be sent wirelessly and not interfere with each other. These will involve hardware and software techniques. So, to start somewhere, could start with multiplexing techniques..... like FDM, TDM, etc. The systems used these days generally involve spreading message (signal) energy around a selected band of frequencies in some prescribed way. And the receiving side will be able to recover the message signal (since it is assumed that it is able to undo what was done at the transmitting side ---- plus taking into account practical effects, like effects added between the transmitter and receiver side).

afedorov:
In particular, I want to understand:
how shield understands that message is received?
what if there are two transceivers? How does shield know that there are two messages in the air?

If you're talking about packet-based RF communications, every packet has a preamble sequence at
the start which allows the receiver to distinquish it from noise (though occasionally an RX will make
a mistake - the packet checksum will catch that). The preamble typically uses a different coding scheme
from the packet contents so it cannot be confused with the middle of some other packet.

If two packets are in the air at once, typically the RX only sees one of them (if its FM, the stronger
signal tends to completely mask a weaker one). Other modulation schemes may be more likely
to see a packet collision as corrupt data. Basically packet delivery is never guaranteed.

Usually packets don't collide because they take a few milliseconds and aren't being sent at high
rates, unless we are talking about high datarate protocols like WiFi, in which case the timing of
packets is tightly policed between the various nodes on the network.