What platform you develop on, and how to use browser as interface?

As somebody just learning to code, I have a couple questions for the developers that I am curious about:

Q1
What platform/language are you using to develop Duplicati for cross platform deployment?

Q2
How do you use a web browser as the GUI interface? (This one intrigues me most!)

Thanks

It was written in C#. Here on Github you can find the main Duplicati project as well as the web server piece:

HTML and JavaScript, probably not that much different from a typical web site you use.
A human-friendly GUI runs a computer-friendly HTTP interface, which you can see with
web developer tools in your browser (commonly F12) which actually controls Duplicati.

Duplicati Client is a third-party tool that goes right to internal server interface from a CLI.
That was reverse-engineered. I can’t explain it all, but sample POST processor is here.