Indexers are search engines for usenet, just like trackers allow for searching for torrents. You’re curently using torrent trackers through Torrentio, which downloads the torrents through RD. RD then streams the media to you. This is convenient but as far as I know only works with public trackers, since most private trackers require you to directly upload as much as you download.
Yes, the *arr apps download the requested media through torrent or usenet. They usually run on a server (old laptop, RPi, …). They allow for searching and then automatically downloading content in the right resolution, language and size through torrents or usenet. This downloaded media might then be watched through Jellyfin or Kodi.
If you use torrents directly (public or private), always use a VPN in Germany. Downloading through usenet isn’t a big risk without VPN, just like torrents with RD isn’t risky either. Using *arr with torrents means a VPN is necessary to avoid unfriendly letters.
So, is the goal here is to replace Torrentio and hook it up to stremio by itself?
I don’t know whether Usenet works with Streamio but there might be a plugin. It might be that Streamio just doesn’t work well for german content.
Sorry, it’ll be difficult beating Streamio + RD in ease of use and cost. I’m using *arr and Jellyfin which means I have a server (desktop pc) with a big HDD running 24/7. Since I’m using usenet the cost of indexer and provider comes on top.