How do true wireless earbuds perform inter-ear communication?
I can't stand not knowing how this works so I did some research by reading some patents - their language is intentionally vague in some areas, but I did discover some commonalities in what they considered to be current normal practice:


It seems common in more recent true wireless earbuds for the "slave" earbud to receive its audio data not through the master re-transmitting to it, but by eavesdropping in on the bluetooth connection from the source to master.  This avoids the need for re-transmitting the data.  The slave listens on this connection but never replies; that is left to the master and the source device does not even need to know that there are two earbuds listening to it.


This approach seems more efficient and it seems like it would cut down on latency, yet true wireless earbuds continue to have significantly higher latency than other stereo bluetooth earbuds that are not true wireless.  I can speculate that additional latency still needs to be built in for them to be more tolerant to congestion throwing sync between the two earbuds out (for example, slave fails decoding a packet and needs it re-transmitted, so it must ask master to ask the source to re-transmit it, and this all is needed before the respective piece of audio is played).  But another explanation is that common true wireless earbuds don't use this practice yet.

For synchronization and state information the two earbuds still need to communicate with each other as well.  I've seen this referred to as a "secondary" connection and as a "hybrid" connection (the source calling it a hybrid connection referred to the slave earbud's passive listening to the original connection a "secondary" connection).


This connection between the two earbuds appears to be a classic bluetooth connection.  This doesn't rule out some devices using more exotic methods of communicating with each other, but this would make sense given that both earbuds are already bluetooth devices.

In online articles and product marketing information I've seen reference to some bluetooth devices that use "near field magnetic induction" to communication with each other.  As far as I can tell this is marketing lies.
In any case, in a true wireless stereo pair the master is still doing more work than the slave requiring greater power use; some these days can swap master and slave without any interruption to the audio in order to drain battery more evenly, leading to greater battery life since your battery life of the whole pair is only as good as the master's remaining battery.  I believe this functionality is starting to appear in some products but isn't that widespread yet.


The whole concept of true wireless earbuds is so new that almost everything published about it is from 2016 or later, and the devices back in 2016 and 2017 were primitive by today's standards.