In the past, we used to run non-intrusive display ads through various networks. Because of the poor quality, we stopped doing this.
Right now, we run even less intrusive ads (visible on the top of Discover) through AdEx (https://www.adex.network/), which is a company by the same founding team that focuses on privacy preserving, transparent ads.
Depending on your jurisdiction, you may not see that many third party ads, but you should have seen some at some point.
Furthermore, the team behind Stremio also handles software development for AdEx, which is how it gets further income. However, there are 5 dedicated Stremio developers, two of them working on QA and improving dev experience of addons, and the rest sharing duties between our upcoming open-source web version, the Android app and the desktop app.