Cordova is an open-source framework to convert Onsen UI apps to native webview-based apps running locally on iOS and Android.
Monaca is the toolset, made by the same team, that helps Onsen UI developers to build hybrid apps. For instance, Monaca CLI and desktop GUI tool called Localkit can be used to start a project, debug the app on the device, and perform remote build. We also have cloud IDE. Monaca has tiered subscription options including free plan.
Progressive Web Apps (PWA) brings a new experience to your mobile apps. PWA is a Web app with a manifest file and a background-running Service Worker. By adding those, you can receive push notifications, offline support, and additional features to give your mobile apps even more power.
Onsen UI is also suitable for developing web apps with any screen size. Onsen UI components are responsive and scalable, fitting perfectly even with desktop websites. Furthermore, you could create a desktop app by using Electron or Node WebKit.