Retention refers to how long a particular provider keeps the data users upload. 3-5k days is pretty typical, but there are some lower ones. Data is also mirrored across the backbones of all the different providers; so if it's removed from one (due to retention or a takedown notice) it's still available on others.
I've had little to no issue finding content, with 97% of data I've requested being available (stats from SabNZBD); but in the off chance you want something that is unavailable, most indexers have a requests section.
Similar to setting up torrenting, usenet indexers/clients can be added to the arr stacks for automation. I'm not sure about Kodi/Real Debrid as I don't use those.
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