Thank you. The problem is being solved and finished.
This last question: for the sake of being native in both functioning and appearance we are recommended to code the appearance, (what we see when working with the app, front-end) using QML and code the real work in C++ (back-end). Is it right.

I don't use Twitter and don’t know how it looks.
I don’t mean a specific app; I mean QML apps to be as modern in both functioning and appearance as though they’re created by swift and java.

I didn’t get “for crunching numbers”, did you mean “for speed/performance” or “to look more native”

Also I meant “comparable”, “compatible” was a typo, sorry.

Now, do you think if the programmer is expert or skillful well, he can create Android and iOS apps using QML as perfect as the ones created by Java and Swift, respectively?

What do you mean by "to be compatible with other similar apps" ?

The goal here is to share as much possible code between platforms and then only specialise a few bits if needed.

Design: it depends on your app and what it does. You have to be more specific.
Speed: You will usually build your GUI with QtQuick and for crunching numbers use C++.
Doing the task: this will depend on how you implement said task.

QtQuick provides a set of control that should look native so your application should look native on all platform you are targeting. You can create your own controls when needed.