For sensor decoders please go to https://github.com/Milesight-IoT/SensorDecoders where you can find all LoRaWAN sensor decoder examples in Javascript.
For the decoder/encoder files end with Chirpstack, please copy them to Chirpstack server or UG series LoRaWAN gateways.
Chirpstack V3:
UG Series gateway:
The Thing Stack: