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RTSP is used for third party storage transfers. https://support.apple.com/en-us/HT210538 for iCloud storage answers.
Sorry, there was a problem. Please try again later.Can't answer the Homekit part of your question, just switched back to Apple and not using Homekit much. However, the answer with the Synology NAS, is you can absolutely use the NAS. I am not using this feature as the Eufy base unit has been able to store plenty of video off of 3 cameras. Regardless, the Eufy can use the RTSP protocol which Synology supports.
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