Hey, sorry it took so long to see your question. Here is a paper (PDF) on the subject with diagrams.

https://www.research-collection.ethz.ch/bitstream/handle/20.500.11850/42365/eth-4572-01.pdf

The link is composed of two parts, the emitter and the receiver. The emitter captures the LF signal and up-converts it to 2.5 GHz. The obtained 2.5 GHz signal is then amplified and transmitted over the air. The receiver part of the link receives this signal and down-converts it to obtain the original LF signal. This LF signal is then amplified again and sent to a loop LF antenna which reproduces the signal that was emitted by the car in its integrity.

Edit: and here is a times article that covers the problem in one area. https://www.nytimes.com/2024/02/24/world/canada/toronto-car-theft-epidemic.html