All Qt resources, including the installers, are available at https://qt.io. To obtain the open source version of Qt, go to https://www.qt.io/download-open-source/. The page suggests the online installer for your current operating system by default, as shown in the following screenshot. Click on the Download Now button to download the online installer, or click on View All Downloads to select a different download option:
When the download is complete run the installer, as shown:
Click ...