Stremio is basically just a foss streaming app that doesn't do much and relies on customization through extensions. There are extensions that add streaming services and extensions that grab recommendations/popular series & movies from different sources. The basic idea is, that you should connect all your streaming services to it (through extensions) and than add stuff like rottentomatoes and IMDB to it, to get recommendations. Then you select or search for whatever you want to watch and Stremio shows you, which of your connected services has that particular stream available. Technically completely legal and fine.

However, it's often (ab)used for streaming directly or indirectly from filesharing protocols. So, instead of adding Amazon Prime or Netflix extensions to it, which discriminate against FOSS clients by only delivering low-quality streams, you could add something like Torrentio. Torrentio is an extensions that can use various torrent trackers (both free, insecure ones, like 1337x or Piratebay, as well as private trackers) to look for available torrents for the thing you want to watch and then it acts as a torrent client and streams the video file directly from that torrent.

Torrentio can also be set up, to use a Debrid service. A Debrid service, is a cloud service that acts a bit like a cloud storage that can download files from torrents or filesharing sites (often through a premium subscription at that service, that you don't have to buy yourself) and then allows you to download these files from the cloud storage. Most importantly, it decouples you from the torrent, acting a bit like a proxy or VPN, and keeps your IP safe by doing that (e.g. to avoid law enforcement). It also allows you to start a torrent download that has few seeders and then let it run possibly for weeks and get notified, once your Debrid has completed the download. And lastly: a good Debrid service basically 'seeds' the files other people have downloaded on that service to all other people on the service, that request the same file. (We call that 'cached') Through that, you can get most popular torrents in a matter of seconds, which effectively allows you to stream them fast and reliably, like from any streaming platform.

Again, Torrentio can use a Debrid service like that, which makes it secure, reliable and convenient. A really good Debrid service should cost around 3€/month and will have basically all films and shows from all big streaming platforms (Netflix, HBO, Prime, Disney,...) already cached in 4K HDR+ Dolby whatever. This basically offers you a much better service than any other streaming platform, doesn't serve you ads, preserves your privacy and does all of that for the small cost of 32€/year and living in a legally dark grey area.

Torrentio has a small flaw, when used with a Debrid: Torrentio uses torrent trackers (1337x, NYAA, EZTVx...) to find a fitting torrent and then requests that from your debrid service. If you are lucky, your service has that torrent cached and you immediately start streaming. Otherwise, your debrid has to start downloading the torrent and will only serve it to you a while later.

An alternative extension for Stremio is Debrid Media Manager (DMM). It basically tracks, which files are cached on your debrid already and then allows you to specifically request those already cached files, making it start the stream more reliably. However, most debrid services don't really like having their cached files tracked, for legal reasons, and sometimes close or obfuscate the API, which makes DMM a bit less reliable on some debrids. Then again, the Torrentio service that downloads tracker information also likes to fail sometimes.

I have both installed (using the same debrid) and default to DMM and only check Torrentio when DMM fails. Other people have it the other way around.