Can bus remote start tap in, plus tx and rx question.

I'm wondering if anyone can explain to me what the TX and RX wires that need to be tapped into when installing a modern remote start device do, what protocol they run on, and what that date actually is? My guess is it has something to do with the car's immobilizer and maybe is a copy of the chip-key data? Is there a good way to tap into it and read it and decode it?

I think that is very vehicle specific. For example, here are all the various vehicle harneses available for one particular iDataStart module:

https://www.crutchfield.com/p_794ADSHCX/iDatastart-CMHCXA0.html