Stremio comes with a few addons for content navigation by default, but to actually watch most things within Stremio itself, the unofficial Torrentio addon is needed. There's other content addons too, but Torrentio seems to work the best.
Torrentio itself needs a debrid service API key to function, with Real-Debrid being the cheapest at about $3 a month.
Careful with Stremio and Torrentio. I forget the details but one of them isn't working with real-debrid anymore. Just do some research before getting a subscription with real debrid.
It works with real-debrid, there's just fear that real-debrid's content will be reduced by the outcome of the latest lawsuit. I've switched to TorBox because of that (and the lower price), but I honestly haven't noticed any degradation of the real-debrid service yet.