Qt Designer is included in Qt sources and works only for UI files. Qt Creator (the IDE) is a separate application and a separate project. You need to install or compile it first, for example from qt.io/downloads.
Once you have Qt Creator installer, you need to open the settings, add your Qt Version, then add a so called Kit (which is Qt Version + compiler bundle), and then you are ready to go.
If you want to avoid all that hassle, use the Qt installer from the link above - it will install Qt, Qt Creator, and set everything up for you, completely automatically.