Even by 1999, when Qt 2 was released, the new "Unicode-aware" QString class used UTF-16 internally, not UTF-8. This remains the internal Qt encoding even today

Even by 1999, when Qt 2 was released, the new "Unicode-aware" QString class used UTF-16 internally, not UTF-8. This remains the internal Qt encoding even today (see https://wiki.qt.io/QString#Unicode_methods_in_QString https://wiki.qt.io/QString#Unicode_methods_in_QString), although the support for UTF-8 has improved substantially, to the point that Qt assumes that 8-bit strings not provided an alternate encoding are UTF-8, that 8-bit filenames can be expressed as UTF-8, and so on (see https://doc.qt.io/qt-6/unicode.html https://doc.qt.io/qt-6/unicode.html).