The source packages for Qt 5.15.2 are hosted on Qt’s public archive:
What is your (Windows, Linux, or macOS)? download qt 5.15.2 offline installer
Check the tab to ensure the offline path to qmake is correctly auto-detected. Create a basic "Qt Widgets Application" project. The source packages for Qt 5
Maintain older codebases that are not yet compatible with Qt 6. Method 1: The Official Qt Open Source Archive (Recommended) download qt 5.15.2 offline installer