That’s how the app tool works by launching Win32 Desktop App that contains an embedded Windows x64 build CEF (Chromium Embedded Framework) browser.
Whilst the app tool only supports Windows (embedding a CEF build per platform isn’t feasible) the Sharp/Vue Desktop Apps are cross platform in that they can run a headless version of the app with the cross-platform x dotnet tool which launches a headless version of the App (i.e. the dotnet server running) with the OS’s default browser.
If you didn’t need to run C# the obvious solution is to use Electron, but if you did that’s not going to be optimal as it already bundles a node server runtime that is used to render the App, you’d have to bundle it with an additional .NET Server whose lifecycle is tied to the App.
If the headless UX provided by the x dotnet tool isn’t satisfactory the only x-plat solution AFAIK is probably Blazor Desktop App which I assume is due to be released with .NET 6.