nvr is a wrapper around openRTSP, which is a required dependancy, that allows you to manage multiple camera streams via JSON based config file.
nvr will run for a specified period of time, ususally 24 hours, and write out new video files at a specified interval. Generally speaking you would create a cron job that starts nvr once a day, and it will run for 24 hours, then be restarted. This is primarily to name the video files by YYYY-MM-DD-interval.mp4
nvr does not do any file or disk managment, you must set up appropriate cron jobs to remove old files and monitor disk usage.