To be fair the site claims they have an open source application yet there is no link to the source code.
Another commenter mentioned github so I searched around & found a potential github project but they're at 4.0.0 release while this site you linked claims to offer a "4.2.0.apk" so I guess it's some random 4.2.0 release version?
Are you able to link the source code that this website refers to?
EDIT: Nevermind I think I found it, the .apk being offered matches this project at github https://github.com/recloudstream/cloudstream/releases though I'm not too sure if the .ws site being linked here is associated with the github project (the github page doesn't mention the .ws site and the .ws site doesn't mention the github page)