I presume CSMA is used? Could also consider TDMA, or something (excessively?) fancy: “CRAM: a time-synchronized cryptographic frequency hopping MAC protocol designed for the LoRa physical layer” - https://dl.acm.org/doi/10.1145/3557048

Carrier detection is complicated by signals being below noise level, so checking RSSI is not sufficient. Channel Activity Detector (CAD) is used, which is a feature of Semtec LoRa radios.

https://lora-developers.semtech.com/documentation/tech-papers-and-guides/channel-activity-detection-ensuring-your-lora-packets-are-sent/how-to-ensure-your-lora-packets-are-sent-properly/

It looks like Meshtastic uses this to implement its own MAC in RadioLibInterface.cpp.