OpenSource is free to use for other projects, but not necesesary without to pay for it, there is a lot of OSS which is only freemium or paid soft. In technologic projects (f.Ex.Robotics) all are paid products. For Example the anti-Fingerprint extension Trace is OpenSource, but has a paid premium version with some more functions and filters. Also ProtonVPN and Mail are OSS freemium https://protonvpn.com/blog/open-source/