Welcome to the community Malcom!
Depending on your app idea, why make a native app for it??? Why not just make it a website?
For native android development you need to use Java to Kotlin to develop an app.
For iOS you need Swift.
For websites, you have a lot of options. .NET core and C# is one option but there are lots of others like React, Vue, Angular, etc.
Windows desktop application would be .NET/C#, using WPF or others.
If you want to make a cross platform app (write a single code base for android AND iOS) you also have more options. React Native (note this is not native app development, this is cross platform), Xamarin, Ionic, Flutter etc
Which prototyping tool, again that could be anything, Figma, Sketch, Adobe XD, etc. a prototyping tool is really to get a design and sometimes some basic (or advanced) animations how you want the app to look and function, NOT actually building the app.
Your ideas of wanting a website, desktop, and mobile apps, this is a very large project and you need to learn the basics of a lot of development before taking this on.