Write your app in pure Python. Deploy on desktops and mobile devices.
The main disadvantage is that it does not use the native widget toolkit, although there are projects like KivyMD that attempt to replicate the native look and feel by theming the UI.
As far as I see it, it doesn't exist, on any platform. Every operating system has multiple drawing APIs and multiple frameworks that build on them. The HTML engines are just another framework.
Sharing code is the easy part. The input and output mechanisms and paradigms are completely different, and designs that work great in one context are difficult or impossible to use in another (eg, key bindings or gestures), and hard problems exist in one area that don't in another (mostly on desktop, windows are terrible).