We take DVR as an example, most of them are connected in the same way, except for the different port locations in DVR. Here is how to connect DVR system, we use a DVR as an example:

The VGA or HDMI port of laptop or host of desktop is used to output viedo, not input video, so does DVR's VGA or HDMI port. Therfore, you can't plug DVR to laptop's or desktop host's VGA or HDMI port directly.

The VGA and HDMI port of DVR can only connect to a separate monitor without a host.

Computer can only connect DVR by Internet, not by direct plug. Here is method:

https://hview.kf5.com/hc/kb/article/1032682/?from=draft