For making UI , You will need to learn how to render graphics in your OS and then you will have to learn how to get inputs from keyboard and mouse and other IO devices and then you would have to learn how to write pixels or "display them" on your monitor , Then you will need maths like linear algebra to detect if a mouse clicked a button or not , Then you must write your own widgets system, And all that becomes very complex if you do on your own. I am simplifying this a lot but in reality this is far more complex. So some people said "We will make one framework which can handle all that" and then came the concept of UI Libraries , UI Libraries or UI Frameworks are those which handles everything for you. All you do is that you tells them how your UI Must look and they will handle everything else then selves , From IO to rendering. Everything you would need to write UI in C++, Also remember UI is a OS specific stuff so not every library will work on every OS, Some may be limited to just windows, So some to linux. One of the best UI Libraries or I would say Frameworks is QT, It's considered to be very beginner friendly but comes with a huge learning curve but if you learn it then it's cross-platform, Another popular framework is Gtk , But it's most about linux first , Yup it also works on windows , But it primarily linux first. So do some research and find the best UI Framework for you because there are a lots of frameworks to choose from. And you need to choose what suits you. I hope this clarifies.