The first Whiteboard app we released was a Windows (UWP) app on an Azure back end. This is still the form factor with the richest feature set.
We later added the web app (Web + Teams) which we are gradually evolving. The web canvas will eventually be the single canvas that powers all apps.
In parallel, we are moving the back end to ODSP (OneDrive/SharePoint). This is the right platform to handle the required scale, compliance, and sharing capabilities.
At the end of this transition period all clients (Teams, Web, Windows, iOS, Android, Surface Hub) will run on a single version of the web canvas to support collab scenarios across all devices.
That transition is complex because we need to build up capabilities on the web app to the level of the Windows client (and beyond), while transitioning the back end, and while also supporting cross-device collaboration sessions across different app versions (e.g. Windows and web) and back-ends.