> const A = () => <div>...</div> ; const B = () => <h1><A /></h1>

Yes, an equivalent in QML would look like this if you want it in a single file:

    Component {
        id: a
        Text { text: "foo" }
    }

    Component {
        id: b
        Rectangle { Loader { sourceComponent: a } }
    }

but the language really wants you to have one component per file, so :

   A.qml: 
   
   Text { text: "foo" }

   B.qml

   Rectangle { A { } }

> if QT is the same,

It's not. QML is at its core not a functional language, it's a declarative & reactive language. In my experience, at least 30-40% of QML code does not need any functions.

> There's no magic and no binding/loading.

I'm not sure we are using the same meaning of binding. The "A" token in your example is obviously bound to the "A" variable declared before.

> It also isn't done by inference of an ID (which would be registration).

ids in QML are just the variable names. eg

   Rectangle { 
     A { id: foo } 
     width: 2 * foo.width
   }