The files: assimpsceneimport.dll/pdb (and assimpsceneimportd.dll/pdb) are missing from Qt6 binaires.
"C:\Qt6\6.1.0\msvc2019_64\plugins\sceneparsers" only contains the gltf importer/exporter.

These files are provided in the MINGW binaries but not in the MSVC binaries.

The same story already appears in the past:
https://forum.qt.io/topic/100825/why-is-plugin-assimpsceneimport-dll-not-available-on-qt-5-12-2-for-windows

But it was fixed and working again in 5.15.0 (I have not tested all versions):
C:\Qt\5.15.0\msvc2019_64\plugins\sceneparsers\assimpsceneimport.dll

But the files are missing again in Qt 5.15.2 and Qt 6.1.0.