That's perfectly normal.
There is no such thing as "Cross Platform .NET"
The C# language and the.NET Framework does indeed run on almost all platforms, but that pretty much ends here in terms of "portability".
Anything beyond that is "platform specific"
For instance, building "desktop" applications is either UWP with advanced styling and reactive data binding but only for Windows or GTK# with Mono which is very limited.
For Mobile either Xamarins.Forms for Android + iOS but it's very limited in terms of features, or it's Xamarin.iOS + Xamarin.Android which require platform specific code for UI / Routing and many others OS specific functions.
For Web , the WASM Binary is something like 5MB+ which makes it not reasonable compared to an app written in Plain JavaScript or TypeScript.
There is still a long way to go for C# to be really "cross platform"