This is a very simple HTTP server set up to accept an HLS stream in fMP4 (fragmented MP4) format and relaying it to YouTube as HLS remuxed to M2TS without re-encoding. It's intended to use with the Tubeist iPhone app: https://github.com/Roenbaeck/tubeist.
You will need Python installed with the Flask module. I use this on Alpine Linux where apk add py3-flask installed the dependency. I am sure it can be done with pip in a virtual environment as well, but setting that up goes beyond my Python knowledge. Additionally you will need ffmpeg installed, since it manages the remuxing and relaying to YouTube. Only tested to work with ffmpeg 6.x and later.
There is a tutorial showing how to set up a docker container with the relay here:
https://www.youtube.com/watch?v=Qzq6nCsHt5c
In order for the relay to work you will need a HLS Stream Key from YouTube. In YouTube Studio, create a new stream key and select HLS as the ingest method. This key is read in a header from the client (Tubeist).