The CSVR or SD card onboard option both provide 24/7 video recording. Depending on how many cameras you have and how big the storage of the CSVR is, you may have longer storage capability on the CSVR.
In either case, video is accessed through the Alarm.com app or website, and the oldest video is recycled to make space for new recording.
How much video can be stored depends on video settings as well. See the example guide below:
Cloud clip storage is separate from CSVR or SD card storage. A guide on how cloud video clip storage works is linked below: