Xamarin, QT, Electron, a few variants of React Native. For games, pick you favorite cross plat engine.

Follow the steps at https://sites.google.com/site/easylinuxtipsproject/oldgrub for a free Windows VM development license.

For Apple, it is an entire hardware platform.

Partially, those frameworks don't expose OS specific features.

You can run VMs. On any desktop OS or cloud provider.

depends on what you're developing right? If the libraries are there, and you have a cross compiler, then it'd be fine.

Use a VM as a start.