How does one create a ui?

Hi,

I’m currently learning cpp and py3 and am planning my first software. As I want to make it as user friendly as possible the UI came to my mind. To program it from scratch seams like it would cause too much work.

How is ui created? I found software for mockups and also interactive but how is the final product made?

Can the mockup be turned into actual UI that is installed via an installer?

I really need some guidance.

My course ein cpp nor py3 is covering that topic.

Thanks for any advice. Cross-platform dev would be nice. Espacially OSX and win10.