It's already saved. It records directly to a file. Set the path to that file in Settings -> Output, *before* you start recording.I don't know how to save it
You can load the recorded video file as a Media source, or you can use the Replay buffer in Settings -> Output -> Advanced mode -> Replay buffer. The Replay buffer is in RAM, so you need that much extra available, and it allows you to do things that would be difficult with a file. And you can dump it to a file if you want too.(or to replay it while in OBS for that matter)
sudo apt-add-repository ppa:obsproject/obs-studio
sudo apt update
sudo apt install ffmpeg obs-studio