There are several ways to get the C-ITS frames received by an ESP32-C5 into OpenTrafficMap (i.e. onto our MQTT server):
Connect the ESP32-C5 directly to the internet with an SPI-to-Ethernet chip (W5500, ENC28J60): opentrafficmap/its-g5-receiver-firmware
This is what the official OpenTrafficMap node uses — see Hardware Setups.
Connect the ESP32-C5 to a Raspberry Pi via USB / UART: jstsmthrgk/simple-its-g5-receiver-firmware
Connect the receiver to an Android phone via USB: sascha8a/cits-to-go — useful for mobile reception.
A repurposed AVM FritzBox 3390 can receive 802.11p directly: MPW1412/avm-fritz3390-802.11p-otm together with MPW1412/openwrt-otm-bridge
An ESPHome external component for the ESP32-C5, including an example configuration that forwards frames to OpenTrafficMap: hn/esphome-its-g5 — details on Home Assistant.