Apart from using a plug in or other third party device, is there anyway Unity can support bluetooth communication between two devices?
All of the links I have found seem to relate to third party options which I want to try and avoid.
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The reason you're only finding third party options is because Unity doesn't have any built in support for Bluetooth.
is there anyway Unity can support bluetooth communication between two devices?
Yes, you'll need to use a third party library to add the support, or you can write your own (not recommended).
Which library to use is up to you, find one that meets the requirements for your platform and works with the hardware you want it to.