Adjacent to OBS, I want to give an unsolicited shoutout to VDO.ninja.
It's a very lightweight way to push low latency video between devices via the browser. I've used it to turn my phone into a webcam, to jointly watch movies remotely (with simultaneous voice chat included), to make a multi-person video call, to stream my phone's screen to my desktop, etc All kinds of cool uses.
Of course, it can feed into OBS, which was sort of the original purpose, but it is so more flexible than that. Of those above examples, only using my phone as a webcam involved OBS.
There's a linked series of videos demoing it at the page, if you scroll down.