How OS

Pretty much Qt. You can learn how different OSes have different way.

However, modern GUI programming is happening on the web and mobile phones. React is popular, but it’s too high level and their XML-based approach is horrifically designed. Swift UI is nice, but it’s Apple-only. Flutter is a good middle ground, but its Desktop capability is limited. You’re going to shoot on your own foot several times.

Macs, or iTunes on Windows. There are two modes of a backup: actual one or backup in name only (like the names of the apps, I wish the latter fucks off).