What are the best practices/commonly accepted methods for capturing video data from legacy camera systems? (CCTV, analog, etc)

I have a potential personal project that I'm brainstorming, that involves video data from legacy camera systems already in place. I'm curious to learn, what are the best practices/standard methods to capture video streams from these legacy systems like CCTV, etc so I can work with them in code?

Basically, for any legacy camera that doesn't support a standard streaming protocol (like RTSP or similar) what are the commonly used pieces of hardware, or software, or both, that are used …

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