Crowdsec will analyse the logs from your reverse proxy and identify malicious actors, bots and whatever else you configure it to. It then tells a "bouncer" (part of your reverse proxy) to reject the request, block the IP or redirect them to a captcha.
The downside is that it's kinda a pain to get set up, at least in my experience. It also stopped working without informing me and I've never been able to get it working again since. If you don't use docker the experience might be a bit easier.
Yes, "All" shows all the communities users are subscribed to on your instance (since communities don't federated with an instance until someone on that instance subscribes to it)
I never got the point of this sub on Reddit, outside of the joke "well guess I have to unsubscribe to never broke a bone", but maybe I'm missing something
In addition to checking the official apps player settings, I found Findroid can direct play a lot more files than the official app.