The pre-built Qt Creator (installed by the online installer) is missing plugins\designer\qwebengineview.dll

Workaround
Manually deploy the binaries and resources needed to let Qt Creator load the Designer plugin.

For example, Qt Creator 8.0.2 uses Qt 6.3.2, so the files to deploy on WIndows are:

• C:\Qt\6.3.2\msvc2019_64\plugins\designer\ → C:\Qt\Tools\QtCreator\bin\plugins\designer\
  • qwebengineview.dll

• C:\Qt\6.3.2\msvc2019_64\bin\ → C:\Qt\Tools\QtCreator\bin\
  • Qt6Positioning.dll
  • Qt6WebChannel.dll
  • Qt6WebEngineCore.dll
  • Qt6WebEngineWidgets.dll
  • QtWebEngineProcess.exe

• C:\Qt\6.3.2\msvc2019_64\resources\ → C:\Qt\Tools\QtCreator\bin\resources\
  • icudtl.dat
  • qtwebengine_devtools_resources.pak
  • qtwebengine_resources.pak
  • qtwebengine_resources_100p.pak
  • qtwebengine_resources_200p.pak