OK, starting with mobile internet. My contract is not the best but I still get 15gb per month for just over a tenner. For under £30 a month you can get unlimited data.
A livestream is going to use approximately 1-1.5gb an hour and considering my normal monthly use is considerably less than 15gb I could get 4 hours streaming and it wouldn’t cost me a penny extra in data.
So then it comes down to the camera. You can just mount your phone on your person or bike and use that – but it’s a bit clunky. Folks who do it probably have a gopro or similar. On a gopro 7 or newer the gopro quik app has an option to livestream to the main platforms (facebook, youtube, twitch).
They even have instructions https://community.gopro.com/s/article/How-To-Live-Stream-From-Your-GoPro?language=en_US
You know, I might try live streaming my next ride to see how it works.